An Assessment of Immersion Silver Surface Finishfor Lead-Free Electronics

WEIQIANG WANG,1 ANUPAM CHOUBEY,1 MICHAEL H. AZARIAN,1

and MICHAEL PECHT1,2,3

1.—Center for Advanced Life Cycle Engineering (CALCE), University of Maryland, College Park,MD 20742, USA. 2.—City University of Hong Kong, Kowloon, Hong Kong SAR. 3.—e-mail:pecht@calce.umd.edu

The worldwide transition to lead-free electronics has increased the usage ofseveral lead-free pad finishes for electronic assembly manufacturers, includ-ing immersion silver, immersion tin, electroless nickel-immersion gold, andorganic solderability preservatives. This study assesses and comparesimmersion silver as a circuit board finish in terms of its ease of use, wetta-bility, solderability, shelf life, appearance, solder joint strength, intermetallicand void formation, reliability, and costs.

Key words: Immersion silver, surface finish, lead-free, circuit board

INTRODUCTION

Copper is the most widely used metallizationmaterial for printed circuit boards (PCB). Whilecopper can have good solderability, it oxidizes rapidlywhen exposed to the environment. This oxidationquickly reduces the wettability* and solderability.**

Surface finishes are commonly used to protect thecopper metallization of the PCB from oxidation, aswell as from contaminants and damage from mis-handling prior to assembly.2 The surface finish pro-tects the base material (copper), enabling goodsolderability of the PCB even after a period of storage.Some surface finishes also serve as a barrier layer toreduce interdiffusion between the solder and theunderlying base material, since the base material

may dissolve and diffuse into the solder and formintermetallic compounds (IMCs). These IMCs aretypically brittle and their excessive growth candegrade the reliability of solder joints.3 The forma-tion of an intermetallic compound is a diffusion-controlled process. Some surface finishes inhibitsolid-state intermetallic compound layer growth,which may occur during high-temperature operations.

As the electronics industry has transitioned tolead-free electronics, both to comply with govern-ment legislation and to be compatible with supply-chain infrastructure,2 several surface finishes havebecome replacements for traditional tin–lead fini-shes. The key lead-free finishes are immersion goldover electroless or electrolytic nickel (ENIG),organic solderability preservative (OSP), immersiontin (ImSn), and immersion silver (ImAg).

This paper evaluates ImAg as a circuit boardsurface finish in terms of its ease of use, wettability,solderability, shelf life, appearance, solder jointstrength, intermetallic and void formation, reli-ability, and cost. A comparison is made with othercommonly used lead-free finishes.

LEAD-FREE SURFACE FINISHES

In lead-free electronics, tin–lead finishes havebeen replaced by OSP, ENIG, ImSn, and ImAg.

(Received June 18, 2008; accepted March 6, 2009;published online April 14, 2009)

*Wettability measures the extent and rate of the wetting processduring solder assembly, during which the molten solder spreadsacross the surface to be joined. The board metallization shouldhave good wettability to ensure firm adhesion of the solder to thebase material.

**Solderability describes the ease and effectiveness with which asurface is soldered by a certain kind of solder. Good solderabilitywill benefit the formation of high-quality solder joints betweenthe solder and the base material.1

Journal of ELECTRONIC MATERIALS, Vol. 38, No. 6, 2009 Regular Issue Paper

DOI: 10.1007/s11664-009-0761-z� 2009 TMS

815

Each of these finishes has certain advantages anddisadvantages.4–8

The OSP finish is an organic treatment appliedover clean copper pads, which protects copper frombeing corroded or oxidized. The typical OSP processconsists of, first, acidic cleaning of the bare copper,then microetching, air knifing, applying the OSP,another air knifing, and a drying process. In thefirst step, the acidic cleaning solution removes sur-face oils and solder mask residues from the exposedcopper surface; the cleaner prepares the surface toensure controlled, uniform etching in subsequentsteps. Next, the board is processed through a mi-croetch solution that typically consists of diluteacids such as hydrochloric, sulfuric, or acetic acid.The etching of the existing copper surfaces removesany remaining contaminants and chemicallyroughens the surface of the copper to promoteadhesion. Air knifing then removes excess solutionfrom the panel to limit oxide formation on the cop-per surfaces prior to the OSP coating. Next, a pro-tective OSP layer is selectively deposited on theexposed copper surface in a water and acid bath.The protective layer adheres to the copper to pre-serve the solderability of the copper surface forfuture assembly. Air knifing is then used again toremove excess OSP from the panel and to promoteeven coating across the entire printed wiring board(PWB) surface. Finally, warm-air drying cures theOSP coating and helps to remove any residualmoisture from the board.9

The OSP coating is nearly invisible, and may beapplied either as a thick benzimidazole (0.1 lm to0.5 lm) or thin imidazole (30 A to 100 A) layer. Dueto the thinness of the OSP, the finish may also beprone to mechanical damage (e.g., surface scrat-ches) during board handling operations, whichcould expose the underlying metal and impact padsolderability. Therefore care must be taken to avoidsurface defects.10

The ENIG process involves the deposition of aninitial layer of nickel onto the exposed copper sur-face of the PWB, followed by deposition of a thin,protective layer of gold. The ENIG process generallyinvolves cleaning the bare copper, microetching,applying a catalyst, acid dipping, and electrolessnickel and immersion gold deposition.

The cleaning and microetching steps in the ENIGprocess are the same as those in the OSP process.After cleaning and microetching, the board isexposed to the catalyst, which often consists of apalladium salt in an acidic solution. Palladium ionsare deposited onto the surface of the PWB in a dis-placement reaction, effectively exchanging the sur-face copper atoms for palladium atoms, forming acatalytic layer for subsequent nickel plating. Theacid dip, usually a dilute sulfuric or hydrochloricacid, removes any residual catalyst to prohibitplating on the noncopper surfaces of the PWB. Oncethe catalysts are in place, an electroless nickelsolution is used to plate a layer of nickel onto the

surface of the catalyst-covered areas in a high-temperature, acidic bath. The electroless nickelsolution contains a source of nickel ions and phos-phorous-containing reducing agent, usually sodiumhypophosphite.11 In the presence of the catalyst, thereducing agent provides electrons to the positivelycharged nickel ions, causing the reduction of thenickel and the deposition of elemental nickel ontothe exposed catalyst. Phosphorous is co-depositedwith the nickel with a concentration typicallybetween 2% and 15% by weight, and the resultingnickel–phosphorous alloy forms a corrosion-resis-tant layer that protects the underlying copper.12,13

The thickness of the nickel layer of PCBs is typically3 lm to 5 lm. The immersion gold plating bath thenapplies a protective layer of pure gold onto thesurface of the nickel. A chemical displacementreaction occurs, depositing the thin layer of goldonto the metal surface while displacing nickel ionsinto the solution. Because the reaction is driven bythe electrochemical potential difference between thetwo metals, the reaction ceases when all of thesurface nickel has been replaced by a gold layer,typically with a thickness of 0.05 lm to 0.2 lm.9

The process of ENIG is more complicated andexpensive than OSP since it involves two separatedeposition steps, each of which employs a preciousmetal, a catalytic process, and a chemical reactioninvolved in the deposition of nickel and gold layers.Gold is considered an excellent choice as a surfacefinish since it dissolves readily into the solder anddoes not easily tarnish or oxidize. Since copper has atendency to diffuse into the gold, reach the surface,and oxidize to form an unsolderable surface, thegold layer cannot be plated directly onto copper.Therefore, nickel is used as a barrier layer betweenthe gold and the copper, since it plates easily ontocopper and the gold plates easily onto nickel. Nickelis unsuitable for use as a surface finish layer, sinceit oxidizes quickly in air to form an unsolderablesurface.

Because the immersion gold layer is very dense, itoffers solderability protection similar to the thickerelectroless and electrolytic platings. The gold layerprovides a shield to stop the nickel from oxidizing inthe air and offers an excellent surface for retainingsolderability. ENIG is good for fine-pitch productapplications.14 However, black pad and embrittle-ment of gold intermetallic compounds have beenreported as a threat to the reliability of electronicsassemblies, causing brittle failures at the inter-face.15 Although it has been claimed that a concen-tration of gold in a solder joint below 3% by weight isa good rule of thumb for eliminating the possibilityof gold embrittlement, brittle fracture failures wereobserved when the gold concentration was belowthat threshold.16,17

If an electroless plating of palladium (Pd) isapplied between the electroless nickel plating andthe immersion gold plating, then the finish willbecome Ni/Pd/Au (ENEPIG), which is also being

Wang, Choubey, Azarian, and Pecht816

used in the industry. The electroless palladiumprocess is similar to that of electroless nickel.The palladium layer reduces the formation of blackpad. However, the palladium layer does not preventthe interfacial brittle failures of solder joints in four-point bending.18

The immersion tin process utilizes a displacementreaction between the copper surface and the tin ionsin the solution to deposit a layer of tin onto thecopper surface of the PWB. The ImSn process gen-erally involves cleaning the bare copper, micro-etching, predipping, immersion tin deposition, and adrying process.

After cleaning and microetching, as in the OSPprocess, etched panels are then processed through apredip solution that is chemically similar to the tinbath, thus protecting the plating bath from drag-inchemicals from the previous etching process. Thepredip is designed to activate the copper surface fora uniform finish and help with the adhesion of tin tothe copper surface. The heated tin bath for immer-sion deposits a thin layer of tin onto the exposedcopper circuitry through a chemical displacementreaction that deposits tin ions while displacingcopper ions into the plating solution. The bath isconsidered self-limiting because plating continuesonly until all the copper surfaces have been coatedwith a tin deposit. The presence of a complexingagent, thiourea, alters the chemical activity of themetals, tin and copper, in the solution, and thusenables tin to be deposited onto the copper surfaceby displacing copper.19,20 Water rinses follow eachof the steps described above, with the exception ofthe predip. Following the tin bath, a drying processremoves any residual moisture from the board toprevent staining and ensure high metal quality.9

A typical ImSn finish thickness is from 0.76 lm to1.27 lm.21

Since tin is a major component of lead-free sol-ders, the metallurgy of the ImSn finish is well suitedto lead-free applications. Immersion tin is good forfine-pitch product applications.14 However, tinwhisker formation can occur on the surface of puretin, which may cause reliability risks to the elec-tronics.22,23 Furthermore, ImSn can attack anddissolve solder mask materials easily to undercutcopper traces.21 The formation of Cu-Sn IMCsbetween the copper pad and the tin finish willconsume the tin finish. So the excessive growth ofCu-Sn IMCs will degrade the wettability andsolderability of the ImSn finish.24,25

CHARACTERISTICS OF THE IMMERSIONSILVER BOARD SURFACE FINISH

Immersion silver has emerged as an alternativefinish for lead-free assemblies. The silver layerserves as a protective finish, thereby ensuring thesolderability of the underlying copper. As a protec-tive finish, the ImAg finish coating is incorporatedinto the molten solder at the time of assembly

because of the dissolution of silver into the tin-basedsolder alloys. In the soldering process, the moltensolder wets and spreads over the silver surface fin-ish. Then the silver layer begins to dissolve into themolten solder. Once the silver layer is completelydissolved, the molten solder wets the underlyingbase metal surface, which is usually copper.3

An ImAg finish is also highly solderable, and itssolderability is maintained through multiple reflowcycles. The contrasting colors of silver and coppermake it easy to inspect for defects. For incominginspection on the assembly line, discolored silverindicates that the boards have been mishandledafter plating, revealing degradation of the layer.ImAg is suitable for fine-pitch product applica-tions.14

ImAg is particularly attractive as a finish forhigh-speed applications. At high frequencies, signalpropagation is concentrated at the surface of aconductor. This phenomenon is known as the skineffect. The skin depth, which refers to the thicknessof the conductor within which approximately 63% ofthe signal is contained, is inversely proportional tothe square root of the frequency.26 For example, at5 GHz in copper the skin depth is less than 1 lm,demonstrating why the thickness and resistivity ofthe conductive surface finishes are important forhigh-speed signal propagation. The electrolessnickel layer in ENIG has a resistivity about 50times higher than copper, and tin has a resistivity10 times that of copper, whereas the resistivity ofsilver is lower than that of copper. A typicalimmersion silver layer is thinner than either ENIGor ImSn finishes and is a small fraction of the skindepth even up to 10 GHz. Thus signal losses arehigher on boards using ENIG and ImSn, whileimmersion silver contributes little to signal loss.27,28

Since OSP is an insulator, signal propagationthrough traces with an OSP finish travels throughcopper. However, although OSP has little effect onhigh-speed signal loss, its insulating properties maypreclude its use in applications that require con-ductivity of surface contacts.

In solderless press-fit connection applications,ImAg also shows good performance, although notthe best. To maintain stable electrical performanceunder various environments, a minimum retentionforce is required for a press-fit connection to provideadequate metal-to-metal contact during the ex-pected life of the connection. The performance of sixplated-through-hole (PTH) finishes [hot-air solderleveling (HASL), ENIG, ImAu, ImAg, ImSn, andOSP] were investigated by comparing the retentionforce in solderless press-fit connections. Three PTHfinishes (ENIG, ImAu, and OSP) were at the lowerend of retention force, and the other three PTHfinishes (HASL, ImSn, and ImAg) were at thehigher end. Among all six finishes, the ImSn finishprovided the highest retention force for the compli-ant pin connections, and it demonstrated excellentreliability.29,30

An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics 817

General Plating Process of Silver

The ImAg process involves cleaning the barecopper, microetching, predipping, immersion silverdeposition, and a drying process. In the cleaningprocess, an acid cleaning solution is used to removesurface oils, oxides, and any organic residues left bysolder mask application. The cleaning processensures uniform etching afterwards by providing aclean, consistent copper surface. Then the microet-ching removes any chemical contamination andmetal oxides by lightly etching the exposed coppersurfaces, including the vias and barrels. Etchedcopper surfaces are processed through a predipsolution prior to silver deposition to remove anysurface oxidation that may have occurred in theprevious rinsing stage. The predipping also pre-vents the chemical residues from the previousrinsing steps from getting into the silver bath bydissolving them into the predip solution, which ischemically similar to the silver bath. The silverbath, which is a pH-neutral solution, selectivelydeposits a layer of silver of a certain thickness ontoall of the exposed copper surfaces of the PWB.The plating reaction proceeds by displacing copperfrom the surface with silver. The free copper ionsreleased from the reaction do not inhibit the bath�seffectiveness as the copper concentration increases.The plating process will cease automatically whenthe entire copper surface has been plated with acertain thickness of silver.13 Finally, a drying pro-cess removes any residual moisture from the boardto prevent staining and to ensure metal quality.Compared with the ImSn process, ImAg needs ashorter immersion time. To deposit a thickness of0.5 lm, the ImAg process will take around 1 min to4 min, whereas an ImSn process will require alonger dwell time of 10 min to 15 min, which makesthe conveyor line longer and more expensive.9

For optimum coating performance, the silverthickness has to be in a certain range. The IPC-4553standard31 recommends a thickness in the range of0.07 lm to 0.12 lm for an immersion silver surfacefinish in general-purpose applications.32 An ImAgfinish that is too thin cannot satisfy shelf-liferequirements during storage, while an ImAg finishthat is too thick increases the risk of brittle solderjoints in lead-free soldering.31

Recently, new immersion silver processes utiliz-ing mildly alkaline baths were developed by Char-termate Electronics in Hong Kong21 and Technic inthe USA separately. Chartermate Electronics testedthe solderability of their immersion silver and foundthat it was comparable to that of acid immersionsilver and ENIG, even after heat treatment for 4 hat 155�C, and better than acid immersion silverafter 8 days of aging at 90% RH and 40�C or afterthree reflow cycles. However, the reliability of thesealkaline immersion silver finishes is in need of fur-ther study.

Manufacturing Costs

Manufacturing costs related to plating pad fin-ishes on boards depend on several factors, such asthe materials and processes involved. The USEnvironmental Protection Agency8 evaluated andcompared the costs involved with several pad fin-ishes, including ENIG, ImSn, ImAg, OSP, HASL,and Ni/Pd/Au. The cost analysis included capital,materials, utility, licensing/permits, production,and maintenance costs involved with each pad fin-ish. Figure 1 displays the costs for each technology,with nonconveyorized and conveyorized processes.The costs for each technology were calculated forprocessing 260,000 surface square feet (ssf) ofboard, the average yearly production at the facilitiesincluded in the study. Conveyorized (C) processes

26300

28700

46900

64700

73800

92400

94200

156000

399000

0 100000 200000 300000 400000 500000

OSP (C)

OSP (NC)

ImSn (NC)

ImSn (C)

ImAg (C)

HASL (C)

HASL (NC)

ENIG (NC)

Ni/Pd/Au (NC)

Pad

fini

shes

Cost per 260,000 surface square feet of board (US$)

Fig. 1. Cost of surface finish technologies (NC, nonconveyorized process; C, conveyorized process).8

Wang, Choubey, Azarian, and Pecht818

were reported to cost less per board than noncon-veyorized (NC) processes for the same technology,because of higher throughput rates.8 Though thecost of ImAg is higher than OSP, which is thecheapest lead-free surface finish, it is half the priceof ENIG. The costs of ImAg and ImSn finishes arecomparable. Due to the additional plating processinvolved in Ni/Pd/Au, its cost is more than twicethat of ENIG.

Wettability and Solderability

Various lead-free solder alloys have good wetta-bility on ImAg surface finishes. In wetting tests,ImAg and OSP finishes were found to wet morerapidly than ImSn and ENIG with lead-free solderalloys, including Sn-3.4Ag-4.8Bi, Sn-3.5Ag-1.5In,Sn-3.8Ag-0.7Cu, Sn-3.5Ag, and Sn-0.7Cu. The wet-ting performance of Sn-3.8Ag-0.7Cu and Sn-3.9Ag-0.6Cu lead-free solder alloys on an immersion silverfinish on fresh boards was not as good as on ImSnand ENIG but better than on OSP in both the reflowsoldering and wave soldering processes.33,34 After85�C/85% RH storage for 12 h, ImAg and ImSnfinishes still wetted Sn-3.9Ag-0.6Cu solder on padsas well as on fresh boards. Immersion silver-fini-shed copper pads showed no decrease in wetting ofSn-3.9Ag-0.6Cu solder after four lead-free reflowcycles, while wetting performance became poorer inthe wetting-bar test after storage at 155�C for 6 hfollowed by two lead-free reflow cycles without sol-dering. Storage at 155�C for 6 h or one lead-freereflow cycle without soldering after storage at85�C/85% RH for 6 h resulted in an obvious wettingdecrease of solder on ImSn-finished pads.33 Thewettability and solderability of ImSn-finishedboards decreased significantly after multiple reflowcycles35; thus ImSn is not suitable for multiple re-flow applications. ENIG showed excellent wettingafter different precondition exposures (155�C for12 h, 85�C/85% RH storage for 24 h, four lead-freereflow cycles and 155�C for 12 h followed by tworeflow cycles), while OSP showed worse wetting thanImAg, ImSn, and ENIG after the above preconditionexposures. In wave soldering tests with Sn-3.5Ag-0.7Cu solder, ImSn finishes showed a greater wettingdecrease than ImAg with more lead-free reflowexposures.33

A team study was performed by various compa-nies to evaluate the wettability and solderability ofalternative finishes, including OSP, ImAg, ENIG,ENEPIG, and Pd-to-SnPb finishes.36 The resultsrevealed that the wettability and solderabilityof ImAg was invariant to the pre-exposure, air- ornitrogen-protected reflow processes, or those com-bined with baking before reflowing, while the wet-tability and solderability performance of the otherfinishes decreased after the pre-exposure. ImAg wasconsidered to have the best overall performanceamong them. ENIG had poorer wetting performancebut better spreading performance than ImAg. These

results on ENIG seemed to contradict the results ofthe study by Arra et al.33

Sandia National Laboratories in the USA hasconducted environmental exposure tests to studythe effect of long-term storage on the wettability ofImAg finishes.32,37,38 The ImAg plating thickness ontest coupons was in the thin range, as designatedin IPC-4553. Test coupons were exposed to theBattelle class II specified environment39 in theFacility for Atmospheric Corrosion Test (FACT),which contains 10 ppb H2S, 200 ppb NO2, and10 ppb Cl2 at 70% RH and 30�C. The contact anglesof Sn-3.9Ag-0.6Cu and eutectic Sn-Pb solder onImAg-finished coupons were measured at a solder-ing temperature of 245�C by meniscometer. Thiswas used as the metric of wettability. A qualitativereliability guideline is provided in Table I.

ImAg-coated coupons were exposed to the envi-ronment in three different ways. In one, the couponswere in an unprotected state. The second testpackaged the coupons in vapor-phase corrosion(VPC) inhibitor bags. The third wrapped the cou-pons first with VPC paper and then packaged themin the VPC inhibitor bags. Exposure time was 8 h,1 week, and 2 weeks. After environmental expo-sure, the solders were soldered onto the ImAg-coated coupons using a rosin-based mildly activated(RMA) flux.

The contact angles of eutectic Sn-Pb solder andSn-3.9Ag-0.6Cu on as-received ImAg-coated cou-pons were 5 ± 1 deg and 28 ± 1 deg, respectively.After aging, the wettability of eutectic Sn-Pb solderand Sn-3.9Ag-0.6Cu solder on ImAg-coated couponsboth showed slight degradation. The wettability ofeutectic Sn-Pb solder was still in the excellentrange, whereas the wettability of Sn-3.9Ag-0.6Cusolder was in the range of good to very good.Combining the inhibitor paper and the inhibitor bagdid not further improve the wettability. There wasno trend of degradation of wettability with longeraging time.38

Vianco et al.37 conducted environmental exposuretests on PCBs with an ImAg finish thickness of0.79 ± 0.11 lm. The boards were exposed to aBattelle class II environment39 for 96 h and 336 h,with and without protection. Copper migration wasobserved on the test boards unprotected and exposedto the Battelle class II environment for 336 h, but

Table I. Relative Wettability Guideline, UsingContact Angle as a General Metric40

Contact Angle Range (deg) Relative Wettability

0–10 Perfect10–20 Excellent20–30 Very good30–40 Good40–50 Adequate55–70 Poor

An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics 819

such migration was not found on the boards pro-tected by VPC inhibitor and moisture-barrier bags.So the Battelle class II environment near the boardswas the primary driving force for the migration.Copper concentration was found in the near-surfaceregion of the immersion silver layer by Auger depthanalysis. The strong correlation between the copperand sulfur signal in the Auger depth analysis indi-cated that it might be a reaction between copper andsulfur from the environment that provided thedriving force for the copper diffusion. The surfacecarbon concentration increased from 60% to 67%(at.%), for nonexposed boards and 94-h exposureboards, to 70% to 75% (at.%) for 336-h exposureboards. Poor solderability was observed for all 336-hexposure boards, both protected and unprotected. Itis possible that the increased concentration of carbonat the surface changed the surface chemistry,resulting in the poor solderability. The protected336-h exposure boards did not exhibit appreciablesurface tarnish, but poor solderability was observed.Thus the degree of surface tarnish was not an ade-quate indicator of solderability for an ImAg finish.37

An ImAg finish can maintain its solderabilityafter 35 h of storage in a Battelle class II environ-ment. A significant decrease in wetting rate wasobserved between 17 h and 25 h of storage, whichmight pose a concern for faster assembly processessuch as waving soldering, hand soldering, and somerework activities.32 ImAg-coated coupons were alsoexposed to a steam environment (88�C, 90% RH) for8 h, 16 h, and 24 h. It was found that the solder-ability of the ImAg coating with the Sn-Ag-Cu alloyis generally insensitive to steam aging treatments,whereas for Sn-Pb alloys, the solderability is moresensitive to steam aging. The 16-h steam agingcondition resulted in a statistically significantincrease in contact angle at 260�C.32

Shelf Life

The solderability of the surface finish and theprotection of the base material by the surface finishshould be retained during storage to ensure thatsoldering can be performed effectively and thatreliable solder joints are produced. Though the sol-derability or wettability of the surface finish willdegrade with time, it should continue to meet sol-dering requirements after a period of storage.

High-temperature aging indicated that silvercoatings could meet a minimum 6-month shelf-liferequirement (of class III hardware, per J-STD-003),but more than likely would remain solderable for upto 12 months prior to assembly.32,37

ImSn cannot provide adequate solderability pro-tection after a few weeks of shelf life. Studiesshowed that both oxide growth on the tin surfaceand IMC growth at the interface between tin andcopper can degrade the solderability of the ImSnfinish.24,25 The combination of temperature andhumidity significantly accelerated oxide growth on

the tin surface, and thus degraded the solderabilityof ImSn. Exposure to temperature/humidity condi-tions of 35�C/90% RH and 65�C/85% RH for 1 weekpromotes heavy tin oxide growth, which leads tosignificant loss in solderability.24 Another factoraffecting the shelf life of ImSn is the IMC formed atthe interface of Sn/Cu, which will consume the Snlayer. Experimental results showed that a maxi-mum thickness of 0.5 lm of tin was converted toSn-Cu IMC after 1 year of storage at room temper-ature, while a layer of 0.72 lm of Sn was consumedafter 4 h annealing at 155�C. However, when thelayer of tin was thick enough to protect the IMCsfrom oxidizing, the solderability could still beretained. So a minimum thickness of the tin layer isrequired to retain the solderability of the surfaceafter a period of time. A thickness of 0.8 lm is suf-ficient for 1 year of storage at room tempera-ture.25,41 The shelf life of OSP can be 6–12 monthsin office-type storage environments (20�C to 30�C,30% to 70% RH), but OSP is relatively permeable toair and degrades quickly with sustained exposure tohigher temperatures and high humidity over pro-longed time periods.3 Due to the chemical stabilityof gold, an ENIG finish has a shelf life of at least2 years,8 which is higher than other pad finishes,such as ImAg, ImSn, and OSP.

Tarnishing is one of the mechanisms that de-grades the shelf life of an ImAg finish. Tarnish is alayer of corrosion that develops over the surface ofmetals as they undergo oxidation.42 An ImAg sur-face readily reacts with sulfur in the environmentand forms silver sulfide (Ag2S). Silver sulfide tar-nishes the surface and is a potential concern forincoming inspection and electrical conductivity. Toprevent tarnishing, industries have developedantitarnish ingredients within the silver bath orthat can be applied in a subsequent step. In order tominimize the associated loss of solderability, ImAgcoatings now contain co-deposited organic inhibitorsthat limit the formation of tarnish-causing reactionproducts.43

RELIABILITY EVALUATION OF IMMERSIONSILVER FINISH

A surface finish can influence the reliability ofPCB assemblies. Under storage or operating condi-tions, contamination on the board, or moisture andcorrosive gas from the atmosphere, may cause cor-rosion of the silver or copper surface. The corrosionproducts can cause shorts in the circuits and thusthe failure of the assembly. Silver is prone tomigrate under electrical fields, which may form aconductive path that shorts adjacent conductors onthe circuit. Since the ImAg finish becomes part ofthe metallurgy of solder joints during the assemblyprocess, the solder joint strength and microstruc-ture will be affected by the ImAg finish. This sectionprovides a review of studies comparing various pad

Wang, Choubey, Azarian, and Pecht820

finishes with respect to corrosion, silver migration,and solder joint microstructure.

Corrosion

When exposed to the atmosphere, silver tends toform a layer of corrosion on the surface, whichmostly includes Ag2S and AgCl.44 This corrosion canaffect the board finish both during shelf storage andunder operating conditions. The corrosion duringshelf storage has been mentioned as tarnish previ-ously in this paper. When silver comes into contactwith copper on ImAg-finished boards, galvanic cor-rosion can occur in the presence of a suitable elec-trolyte. Copper corrodes significantly faster thansilver in a sulfur-based oxidizing environment,because copper is less noble than silver. Water fromthe atmosphere can deposit onto the surface. Sulfurcan dissolve into the water to form an acid electro-lyte solution. A monolayer of electrolyte solution is

enough to promote corrosion.45 During the corrosionreaction, copper serves as the anode, while silverserves as the cathode. Copper is corroded to formcorrosion products, which can cause shorts in thecircuits. A simplified reaction mechanism with ele-mental sulfur is represented in the diagram shownin Fig. 2, which omits possible intermediate reac-tion steps. Other atmospheric sulfur compounds candiffuse into the moisture layer and cause galvaniccorrosion of copper in a similar manner in thepresence of silver. Dendritic structures grew on thecorroded surface during the creep corrosion process.From EDS analysis, it was found that the dendriticcorrosion products were mainly copper sulfide(Cu2S), with a small amount of silver sulfide (Ag2S).The corrosion product appears to have semicon-ductor properties. This means it may not cause afailure immediately after physically linking theelectrodes. Under low-humidity conditions, electri-cal failures that caused a field failure might not bedetected on a PCB, but if the board was exposed to ahigh-humidity environment the failure could reap-pear.46

A higher rate of failures was found on computerswith ImAg-finished boards in an environment con-taining a high level of reduced sulfur gas. It hasbeen claimed that the sulfur gas is the main corro-sion agent, rather than sulfur dust particles. Onefailure was from a computer that was used with claymodels for product design. A high level of corrosion,higher than on gold-plated boards, was found on theboards, as shown in Fig. 3. The clay, which con-tained a high level of sulfur, released sulfur gas intothe atmosphere, promoting corrosion. The heaviestcorrosion was primarily inside the via holes. Thatwas because the exposure of copper was more com-mon in places where incomplete coverage by thesilver finish was likely, such as inside the via holesand at the edge of solder-mask-defined pads.45,46

Cu (+)

Ag (-) Ag (-)

Electrolyte layer

Atmosphere

Cu e- + Cu+

S + 2e- S2-S + 2e- S2-

e-e-

2Cu+ + S2- Cu2S

Fig. 2. Diagram of galvanic corrosion of Cu by S in the presence of Ag.

Fig. 3. Corrosion failure on immersion silver-finished board.45

An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics 821

Veale47 investigated the effect of mixed flowinggas (MFG) testing, according to the ISA-71.04standard,48 on the corrosion resistance of variouspad finishes, such as ImAg, ImSn, ENIG, and OSP.It was found that ENIG and ImAg coatings weremore susceptible to creep corrosion or dendriticgrowth failures than either OSP or ImSn under theconditions. Although it was claimed in the articlethat the environmental conditions represented theISA G3 environment, based on the copper corrosionrate (350 nm/day) provided in the article, itapproximated ISA G1 or Battelle class III condi-tions. The test boards followed the IPC-B-25 SIRtest board design.49 Before the MFG test, the boardswere processed through a double paste-and-reflowprocess using the typical lead-free reflow tempera-ture profile. The gas concentrations of the testenvironment are shown in Table II. The test envi-ronment was maintained at 28�C to 29�C with rel-ative humidity of 75%. The test lasted for 20 days.Four boards of each type were tested. The patternswith 0.3175 mm (12.5 mil) line/space were moni-tored for resistance during the test. All circuits withENIG and all but one circuit with ImAg failed afterthe test, while 37% of ImSn circuits and 25% of theOSP circuits failed.

Xu et al.50 also conducted corrosion resistancetests on PCBs with OSP, ImAg, ImSn, and ENIGfinishes, but in a more severe environment. Thecopper corrosion rate was determined to be 600 nm/s,which represented Battelle class IV39 and ISAclass G248 conditions. The gas concentrations areshown in Table III. The temperature was 40�C andthe relative humidity was 69%. To further simulateoperating conditions, half of the samples were alsobiased at 10 V with a current limit control at 4 mA.An IPC B25 comb structure with 1 oz. of finished

copper was used as the test board design. The widthof the copper lines was 0.254 mm and the spacingbetween adjacent lines was 0.33 mm. The testvehicle was distributed to several major PWB finishchemistry suppliers to be coated with OSP, ImAg,ImSn, and ENIG. Before the MFG test, test boardswere processed through two lead-free reflow cycles.

Xu et al. found that the corrosion on the boardswas primarily due to porosity. Electrical shortscould be caused by fiber-assisted electrochemicalmigration (FAEM) and flaking of conductive corro-sion products. FAEM is electrochemical migrationassisted by fibers from contamination. The inad-vertently introduced fibers absorbed enough mois-ture or corrosive gases to allow electrochemicalmigration to occur over them. Fiber-assisted creepcorrosion (FACC) was observed along the fibers onseveral boards. From the test result, it was foundthat the ImAg-finished boards were more suscepti-ble to corrosion than the other three types of boards.After only 2 days, the silver surfaces on all sevenboards were covered with grayish corrosion prod-ucts, mostly Ag2Sn. However, no electrical failureswere caused by this corrosion. The other three typesof boards were less susceptible to corrosion, butcorrosion-caused failures were found on boards withENIG and OSP. FAEM was found on most of thetest boards: ImAg (four out of seven), ENIG (two outof three), and OSP (three out of five) after 40 days oftesting. The ImAg-finished boards showed FAEMearlier than the other boards. FACC was alsoobserved on some boards. No failures were found onImSn-finished boards after 40 days of testing.

Although ImAg, OSP, and ENIG are preferredfinishes for a Battelle class II environment andelectronic devices with a short expected service life(a few years), they may not be suited for long servicelife (10 years to 20 years) in equipment deployed ina harsh environment, such as Battelle class IV orISA class G2, due to their susceptibility to FAEMand corrosion-caused failures in such an environ-ment. Especially when the environment contains ahigh level of sulfur gas, creep corrosion can occurand failures could result on ImAg-finished PCBswithin a few months. ImSn was the only lead-freePWB surface finish that could survive Battelleclass IV and ISA class G2.

Silver Migration under Temperature-Humidity-Bias (THB) Accelerated Testing

The migration of silver under certain environ-mental conditions can lead to bridging between twoconductors. The mechanism is the growth of con-ductive metal filaments, typically in the form ofdendrites, on a PCB through an electrolyte solutionunder the influence of a DC voltage bias. Once thegrowth of the metal filament has spanned the lengthof the conductor spacing, shorting can occur, leadingto intermittent or permanent failures of the elec-tronic system.

Table II. Gas Concentration for MFG Testin Veale47

Gas Concentration (ppb)

H2S 100NO2 200Cl2 20SO2 200

Table III. Gas Concentration for MFG Testin Xu et al.50

Gas Concentration (ppb)

H2S 1700NO2 200Cl2 20SO2 200

Wang, Choubey, Azarian, and Pecht822

The IPC-3-11g metal finishes data-acquisition taskgroup has worked with Underwriters Laboratories(UL) to conduct testing to determine the silvermigration propensity of silver finish. It was foundthat silver was vulnerable to electrochemical migra-tion failures, but only when deposited as a thick,electroplated silver coating. Immersion silver wasfound to perform similarly to all other commonly usedPCB finishes such as HASL, ENIG, OSP, and tin.51

Zhan et al.52,53 studied the electrochemicalmigration (ECM) of PCB test structures by testingthe surface insulation resistance (SIR). The PCBtest structures were assembled on FR-4 and poly-imide (PI) substrates, with different solder alloysand different surface finishes. The test structureconsisted of 11 pairs of solder pads in parallel, asshown in Fig. 4.

The test structures were exposed to a series oftemperature-humidity-bias (THB) conditions thatincluded temperatures of 40�C and 65�C, humidityof 88% and 93%, and voltage bias of 5 V, 10 V, 20 V,30 V, and 40 V. The surface insulation resistance(SIR) of the PCB board was measured in situ tomonitor the migration of silver. The migration ofsilver on the PCB would result in a drop in the SIR

and would lead to a short when the silver migratesfrom one conductor to an adjacent conductor.According to the IPC standard IPC-TM-650 method2.6.3.354 and JEDEC standard J-STD-004,55 a PCBis considered failed when the resistance drops below100 MX.

The test involved 16 assemblies with differentcombinations of board substrates [FR4 and polyim-ide (PI)], surface finishes (HASL, OSP, ImSn, ImAg,and ENIG), solder alloys (eutectic Sn-Pb, Sn-3.0Ag-0.5Cu, and Sn-3.5Ag) and fluxes (no-clean-rosin androsin), which were tested for 8170 h.53 Among thesecombinations, the board that had the most dendritesgrowing between contact pairs was the ImAg-fin-ished board with Sn-3.5Ag solder and rosin-basedflux. The board material was PI. Nine of the 11conductor pairs had dendritic growth. Permanentfailures occurred after 2430 h. Figures 5 and 6 showthat dendritic growth occurred between the con-ductors. The dendrites bridged the two electrodesand resulted in the degradation of the SIR. Fromthe elemental mapping of the dendrite, as shown inFigs. 7–9, the composition of the dendrite was foundto be silver, which might have migrated from thesolder or ImAg finish.

Fig. 4. Test structure used in ECM study.53

Fig. 5. Optical micrographs of dendrites on PI board with Sn-Ag solder on ImAg pad finishes; 73% of the solder pad pairs (8 out of the 11 pairs ofpads on the board) exhibited dendritic growth.

An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics 823

Solder Joint Strength

The influence of pad finish on solder joint strengthhas been studied by Houghton.56 Houghton con-ducted experiments by pulling the leads of small-outline integrated circuit (SOIC) modules tocompare the strength of the solder joints on severalfinishes. The ImAg finish showed the highest leadpull strengths as compared with ENIG and ImSnfinishes in as-reflowed conditions.

The pull strength of solder joints with differentsurface finishes for quad flat packages (QFPs)assembled with Sn-3.0Ag-0.5Cu solder was studied.The pull strength of solder joints were tested on

as-reflowed samples and after 350 h and 1000 h ofaging at 125�C. Experiments were conducted onsamples with different pad finishes, includingENIG, ImAg, OSP, and ImSn. It was found that themean pull strength of leads after aging variedaccording to the pad finishes, as shown in Fig. 10. Itwas observed that the pull strengths for ImAg andOSP finishes were comparable, whereas solderjoints with ENIG finishes resulted in about a 10%lower solder joint strength for both the solderstested.

Solder Joint Microstructure

The solder joint assembly process involves theinteraction of alloys from the component, board, andsolder. The initial formation of intermetallic com-pounds (IMC) during soldering ensures a goodmetallurgical bond. During the microstructure evo-lution, the IMCs will grow and microvoids may beformed. These can impact solder joint reliability.

Intermetallic Growth

During the soldering process, the ImAg finishdissolves into the molten solder to form IMCs.During soldering with tin-based solders, Ag3Sn isthe only IMC formed between silver and tin,according to the Sn-Ag phase diagram.57 Afterreflow of Sn-3.8Ag-0.7Cu alloy on an ImAg finish(reflow process peak temperature of 244�C), aninterfacial intermetallic layer of Cu6Sn5 will befound. Large Ag3Sn needles or platelets are commonabove the Cu6Sn5 layer in the as-reflowed samples.The presence of the silver above but near theinterface after reflow, and its disappearance orreduction in concentration upon aging, indicatesthat silver is diffusing away from the interface andinto the bulk.58

Fig. 6. Electron micrographs of dendrites on PI board with Sn-Ag solder on ImAg pad finish (dendrites shorted two electrodes).

Fig. 7. Electron micrograph failure site of dendrites on a PI boardwith Sn-Ag solder on ImAg pad finish, showing a dendritic distributionof silver.

Wang, Choubey, Azarian, and Pecht824

Spowage et al.59 found that, after reflowing Sn-Pband Sn-Ag-Cu solders with several different surfacefinishes, ImAg showed an intermetallic growthcomparable to that of ImSn, whereas OSP showedthe highest IMC growth. ENIG showed the smallestintermetallic growth, which verified that ENIG isindeed an excellent diffusion barrier. However, thepresence of phosphorous in the plated layer couldhave an effect on the reliability in certain types ofpackages and will become increasingly undesirableas joint height continues to decrease.

In ball grid array (BGA) packages, after 350 h ofaging at 125�C, the IMC layer thicknesses arecomparable in solder joints with ImSn, ImAg,ENIG, and OSP pad finishes for Sn-3.0Ag-0.5Cusolder. The thicknesses range from 2.5 lm to3.5 lm. This growth is equivalent to the levelexpected after 10 years at 70�C, based on the

activation energy values of 1.18 eV for an ImAg padfinish. After 1000 h of aging at 125�C, the IMCthickness was about 3 lm to 4 lm, as shown inFig. 11.

Interfacial Microvoiding

The solder joint interface has been known to besusceptible to the formation of microvoids. Micro-voids have been previously reported to be caused bythe unbalanced interdiffusion of copper and tin atthe interface between the copper pad and the bulksolder. The migration of copper atoms leavesvacancies on the copper pad side; these vacanciesare not filled by tin atoms due to dissimilar diffusionrates. These vacancies eventually coalesce and formmicrovoids.60 Since this diffusion process dependson the surface chemistries, pad finishes may influ-ence the formation of microvoids at the interfacebetween the solder and the copper pad. The pres-ence of microvoids may lead to crack propagationand thus may affect the reliability.

In this study, the solder joint interface on theboard side of a BGA solder joint was studied for theas-reflowed solder joints, and joints aged at 125�Cfor 350 h and 1000 h. Among the four differentlead-free assemblies, Sn-3.0Ag-0.5Cu solder withdifferent finishes (OSP, ImSn, ImAg, and ENIG),microvoids were found in PI assemblies with ImSn

Fig. 9. EDS mapping on failure site (silver).

Fig. 8. EDS mapping on failure site (silicon).

1.1

1.15

1.2

1.25

1.3

1.35

1.4

ImAg OSP ImSn ENIG

Pad finish

Mea

n pu

ll st

reng

th (

kgF)

Fig. 10. Effects of pad finish on solder joint strength for Sn-3.0Ag-0.5Cu solder assemblies.

0

1

2

3

4

5

6

0 5 10 15 20 25 30 35

Square Root of Time (Hours1/2)

IMC

thic

knes

s (m

icro

ns) ENIG

ImSnImAg

OSPSnPb-HASL

Fig. 11. Effect of pad finish and aging on IMC thickness; BGA(SAC)–SAC solder, 125�C.

An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics 825

and ImAg board finishes for solder joints aged at350 h and 1000 h. It was observed that, at theinterface between the IMC and the copper pad,microvoids occurred after 350 h of aging and tendedto increase in number with subsequent aging. Forlead-free FR-4 assemblies with ImAg pad finishesthe location of voids was found to be between theIMC layer and the bulk solder, as shown in Fig. 12,instead of between the IMC layer and copper pad asobserved with ImSn and OSP.

CONCLUSIONS

Manufacturing costs for ImAg ranked in themedium position among the four lead-free surfacefinishes: comparable to that of ImSn, twice the costof OSP, and less than half that of ENIG. ImAg isgood for fine-pitch assembly and has better high-frequency electrical performance than ImSn andENIG. Although OSP has little effect on signal loss,it is not suitable for applications that requiresurface contact conductivity. ImAg has good per-formance in press-fit connector applications as aPTH finish, comparable to ImSn and better thanOSP and ENIG.

ImAg has good wettability and solderabilitycompared to various lead-free finishes—not as goodas ENIG, but better than ImSn and OSP. The shelflife of ImAg also ranked in the medium positionamong the four. It can provide adequate solder-ability for at least 6 months and up to 12 months.During accelerated testing, an ImAg finish can meetsolderability requirements after four reflow cycleexposures or temperature–humidity storage at85�C/85% for 12 h. However, its wetting perfor-mance degrades after high-temperature storage at155�C for 6 h followed by two lead-free reflow cycles.It can maintain its solderability in a Battelle class IIenvironment for 35 h.

ImAg is a good choice for lead-free electronics thatrequire a short service life, such as a few years, in amildly harsh environment, such as Battelle class II,given the combination of its medium performanceand cost. However, in harsh environments such asBattelle class IV or ISA class G2 environmentsor high-sulfur-containing environments, ImAg is

not an option for providing long service life, due toits susceptibility to corrosion and electrochemicalmigration-caused failures. ImSn is a better optionthan ImAg in these environments. However, the tinwhisker risk needs to be considered when choosingbetween ImSn and ImAg.

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An Assessment of Immersion Silver Surface Finish for Lead-Free Electronics 827