Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q456

An Immersion Silver PWB Surface FinishAn Immersion Silver PWB Surface FinishTo Combat Harsh EnvironmentsTo Combat Harsh Environments

An Immersion Silver PWB Surface FinishAn Immersion Silver PWB Surface FinishTo Combat Harsh EnvironmentsTo Combat Harsh Environments

ABSTRACT

INTRODUCTION

The corrosion of Pb-free PCB surface f inishes in

harsh, polluted working environments has been

increasingly discussed and studied recently. Ever-

improving advances in microchip and memory

storage technology, combined with economic

growth and industrialization in emerging markets

have greatly expanded the reach of what are now

common electronic goods. From this, observations

relating to the performance of such systems in

aggressive and unique environments may not be

surprising. One challenging aspect of this subject

emerges as designers, process suppliers, and other

stakeholders attempt to characterize the corrosion

performance of common surface f inishes with

"industry accepted" corrosion tests. It is being

found that long-used test techniques are not

matching f ield observations.

This paper presents detailed corrosion results for

common surface f in ishes in both " industry

s tandard" and "custom-des igned" corros ion

inducing environments. It is suggested that an

improved understanding of the gaps between f ield

and standardized test techniques may emerge from

this work. Additionally, this paper focuses on

demonstrating enhanced tarnish and corrosion

performance for an immersion silver based f inal

f inish. A detailed review of the coating's conf irmed

functional and assembly performance, attained via

a commercially installed system, is also provided.

The implementation of RoHS legislation prohibiting

the use of lead in electronic products has caused

board manufacturers to transition from lead

containing f inal f inishes to lead free alternatives.

Ernest Long, Ph.D, John Swanson and Lenora ToscanoMacDermid

Waterbury, CT, USA

An Immersion Silver PWB Surface FinishTo Combat Harsh Environments

The most common such f inishes currently used in

the industry are: organic solderability preservatives

(OSP), immersion silver (Imm Ag), immersion tin

(Imm Sn), lead free HASL and electro less

nickel/immersion gold (ENIG). These f inishes were

developed to provide an acceptable solderable

surface for component mounting during board

assembly. When these f inishes were developed,

long term corrosion resistance was not considered

to be a key design criterion. With the ever

expanding use of electronic goods in increasingly

harsh/pol luted environments, part icu lar ly in

industrial settings and also in developing countries,

equipment failure has been observed.

Lead free f inal f inishes, as described above, are

usually thin f ilm deposits and are not especially

robust as defenses against exposure to harsh

environments. This exposure often results in visible

coating degradation, such as tarnishing and,

commonly a form of corrosion known as "creep

corrosion".

(A)

www.hkpca.org 57

Technical Paper

(B)

(C)

Figure 1: Creep corrosion on (A) Imm Sn, (B) Imm Ag, and(C) OSP coated boards.

Investigations have revealed that creep corrosion is

induced, primarily, by the reaction of copper and

sulfur [1, 2], producing Cu S type corrosion product.

It was also found during these studies [2] that the

presence of water and oxygen were very signif icant

factors and the proposed mechanism for Cu S

formation has been given as:

4Cu + O (g) 2Cu O (s)

Cuprous oxide when exposed to sulfur or hydrogen

sulf ide in the atmosphere then reacts as follows:

2Cu O (s) + 2H S 2Cu S + 2H O(s)

In a humid environment H S will dissolve and

2

2

2 2

2 2 2 2

2

�

dissociate into a number of ionic species (HS , S

and H ), and in the presence of oxygen, an

environment is produced which provides both

oxidation and transport for the copper sulf idation

(creep corrosion) reaction to occur.

One of the perhaps unique features of creep

corrosion is its mobility across planar surfaces such

as laminate and soldermask on a printed circuit

board (pcb), which can result in bridging of circuit

features to occur. The corrosion product f ilm is

essentially electrically resistive, however as the

f ilm thickness increases over time this resistivity

decreases, reducing ultimately to a point where

when bridging has occurred, electrical failure

results.

In reality, unsurprisingly, creep corrosion has been

found to frequently occur in high sulfur containing

environments, for example, tire manufacturing

facilities (vulcanization process) and paper mills,

where sulfur is used in the bleaching process,

amongst others.

Factors that likely accelerate creep corrosion in the

f ield:

1. High concentrations of elemental sulfur,

s u l f i d e and /o r su l f u r d i ox i de i n t he

environment

2. High contaminated air and particulate flow

rates across the pcb

3. Water condensation onto the pcb due to

thermal cycling of the equipment in a humid

environment

4. Degraded f inal f inish coating stability to the

high temperature excursions experienced

during the assembly process

5. Galvanic couples being created by dissimilar

metals coming in contact

6. Residues and specif ic materials introduced

via solder pastes and fluxes [3]

- 2-

+

Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q458

In an attempt to study how the various lead free

f inal f inishes behave in these sulfur containing

environments, and to evaluate a variety of

corrosion mitigation practices, classic mixed

flowing gas testing (MFG) has been employed. In

this type of testing, in an effort to simulate the f ield

conditions to which electronic materials are

s u b j e c t e d , t e s t v e h i c l e s a r e e x p o s e d t o

combinations of corrosive gases, maintained at

predetermined concentrations and at elevated

temperature and relative humidity. The Battelle and

Instrument Society of America (ISA) systems [4]

are the most commonly used classif ication scales.

Figures 2 and 3, describe in detail, these gas

combinat ions and c lass i f i cat ion for var ious

environments.

When originally designed, these tests were used to

understand the corrosion that would occur on a

surface f inish after exposure to such environments.

Recently, many have attempted to recreate creep

corrosion using the industry accepted mixed

flowing gas testing. It is after this testing that the

vo id be tween indus t ry s tandards and the

contamination levels in the current working world

result in a disconnect. It has been explained that

the levels of contamination in industrial areas far

exceed those previously decided on the in Battelle

and sister methods such as Telecordia and EIA [7, 8,

9]. Specif ically, it has been determined that the

levels of H S are much greater in these settings.

There is also indication that humidity is much

greater and in some cases reaches condensation on

the electronics as explained by the reactions

displayed previously.

In an attempt to recreate these levels, MFG

chambers have been pushed to extremes. To mimic

aggressive industrial environments, we designed a

test where the Battelle Class 4 was used but with an

increase in the H S to 1500ppb. After 21 days of

exposure, no creep corrosion was observed on any

of the surface f inishes (f igure 4A). At this point, the

need to force a condensation step was desired. Due

to the limitations of the MFG chambers and the

possibility of destroying some equipment, the

pcba's were removed from the chamber at intervals

and placed in a refrigerator to force condensation

2

2

CREATING CREEP CORROSION

Figure 2: Battelle Mixed Flowing Gas Levels

The Battelle Labs' testing is segregated into classes

with increasing aggressivity. Class 1 is considered a

clean off ice environment and therefore, no gas

quantities are assigned to it. Class 2 moves to a

light industrial environment and f inally Class 4

represents a heavy industrial environment [4,5].

Tests can be designed to mimic these levels of

contaminat ion. Another industry recognized

classif ication of air contamination was developed

by the ISA. The ISA uses four levels of severity to

d e s c r i b e e nv i r o n m e n t s . T h e i r m e t h o d o f

classif ication is used to identify the level of

corrosion achieved on a copper or silver coupon in a

given period of time. The same types of gases

displayed in the Battelle tests are monitored in

these environments. The levels span from G1

considered mild through G3 which is harsh and

f inally GX which is considered severe [6].

Figure 3: ISA Classification

www.hkpca.org 59

Technical Paper

on the panels' surface once back in the heated MFG

camber. After 8 days of MFG exposure with

condensation steps every 48 hours, still no creep

was created (f igure 4B).

application.

Through extensive research this team has seen that

not only is moisture needed to create the defect but

the other driving factor is elemental sulfur. After

countless hours of MFG testing and a need for an

internal test we worked through many iterations of

creep chamber testing.

Our experience with tarnish resistance initially

resulted in pcb exposures to very high levels of H S.

We were successful in creating black tarnish over

the immersion silver but we were not successful in

causing the actual migration experienced in the

f ield. With the introduction of high humidity and

condensation, we did see the migration of the metal

though the appearance of the creep was not the

same as that seen in industrial environments. The

creep was spiky in nature and adhesion of the

plated surface was lost [11]. Eventually we moved

to modeling clay which contains 40% sulfur. In this

test, the clay was shredded to create more exposed

surface area, water was added to the bottom of the

jar. Temperature cycling of the jars from 50 C to

25 C during testing results in a thin layer of

condensation both inside the glass jar (f igure 5A)

as well as over the board surface. With time, we

observed a breakdown of the clay. Yellow crystals

were occasionally observed on the clay. Analysis of

this material proved to be sulfur. Through this

observation, we f inally moved to sulfur powder as

our source of contamination in creep testing [11,12].

We have seen that the sulfur powder is transferred

to the surface of the pcb during the testing. The

lightweight powder is carried to the board surface

through the moisture in the condensation steps.

We have found that the "tire factory chamber"

(f igure 5B) which contains chemicals commonly

found in the tire vulcanization process to be the

most consistent in creating the high degree of

creep corrosion observed in the f ield. This test is

2

O

O

Figure 4: (A) Imm Ag 21 day accelerated ISA G3 exposure(B) Imm Ag 8 day accelerated ISA G3 exposure withcondensation

Corrosion particularly that which migrates over a

circuit board was not observed in conventional

mixed flowing gas testing. It did not reproduce the

type or degree of defect observed in the f ield. Even

the introduction of moisture did not result in the

desired defect. Again, it is this disconnect observed

from f ield data and standardized, industry accepted

testing that limits the ability to properly create and

test a mitigation step against creep corrosion.

The IPC 3-11 g committee recently set forth

objectives to better address the issues described

above [10]. Until an industry wide methodology has

been agreed upon that accurately reproduces the

type and degree of corrosion observed in the f ield,

chemical supply houses and end users have to rely

on in-house, more empir ica l test methods,

developed through trial and error [11].

Th i s app r oa ch can re su l t i n deba t e and

disagreement as to the importance of the various

influencing factors affecting creep corrosion. It can

also, important ly, cause dispute as to the

effectiveness of mitigating practices, such as the

influence of board fabrication and design practices

[12], and any chemical treatments that might be

employed, either prior to or post f inal f inish

Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q460

considered accelerated in the sense that it creates

excessive creep on immersion silver in 72 hours.

Through this testing we have seen creep occur on

all f inishes. It can be argued that this test is too

aggress ive when try ing to understand the

subtleties between f inishes.

contact functionality. Both the immersion silver and

its underlying copper must be protected from

corrosion.

With the directives cited directly above, this team

set out with the goal of developing effective and

easy-to-apply silver post-treatment. Any treatment

or modif ication aimed at mitigating creep corrosion

would need to be sensitive to the fact that the

benef its described above are at the very least

retained, if not enhanced. Additionally, the current

gaps, shortcomings, and limitations relating to

industry selection of accurate corrosion test

methods were known up-front. Despite this, our

experience in the development and evaluation of

immersion silver f inishes gave us useful references

and methodologies to build from. The process of

this product development and evaluation is

described below.

The use of substantial organic topcoats on Imm Ag,

for example, conformal coatings, have been

considered as a potential approach to preventing

creep corrosion. These coatings are typically

applied post-assembly where they would not be

subjected to the high thermal excursions of lead

free reflow and wave cycles. However, there are

many associated disadvantages to this approach,

such as, equipment requirements, chemical

handling issues at the assembly house, cost issues,

potential for component damage and surface

conductivity.

Ideally, a more universally acceptable solution is

sought pre-assembly and by necessity would need

to be able to withstand the performance demands

that result, such as the above mentioned high

temperature lead free ref low and soldering

processes. For this reason, traditional, metal

specif ic organic inhibitors such as triazoles and

CREEP MITIGATION

Figure 5: (A) Clay Corrosion Chamber (B) Tire Facotry

Figure 6: (A) SEM of immersion silver (B) after corrosion

From this, one can argue that improvements to a

surface f inish that wi l l dramatical ly reduce

conventional corrosion will in fact, retard the on set

and extent of resultant creep corrosion. Especially,

in the case of immersion silver and creep corrosion,

it has been well documented [13,14] that excessive

tarnish to the thin layer of silver corrodes through

the surface to the underlying copper. This then

creates a copper corrosion. Once the copper is

exposed and moisture is present, the copper will

migrate and hence, creep corrode. If both the

copper and s i l ve r can be pro tec ted f rom

conventional corrosion, the migration of the metal

can be dramatically reduced if not possibly

eliminated.

It is our desire to create a more robust immersion

silver. Our customers have expressed the desire to

keep immersion silver as a surface f inish due to its

superior ease of application, solderability, and

www.hkpca.org 61

Technical Paper

imidazoles are not effective for creep corrosion

mitigation due to thier limited thermal stability.

Other azole based inhibitors, such as thiazoles and

thiadiazoles, that are more thermally stable have

been found to offer some mitigation however,

solderability issues are frequently encountered and

are severe enough to preclude these materials from

consideration.

Self-assembly monolayers (SAM's) of a variety of

chemical types are available and can be tailored to

provide some intriguing coating characteristics.

These metal selective, extremely thin coatings

frequently are not robust enough to endure thermal

excursions such as those imposed by lead free

processing. However, we have found a particular

group of materials that are stable to extremely high

temperatures, giving good tarnish and improved

corrosion resistance whilst also retaining the

appropriate level of solderability performance

required.

A rev iew of the func t i ona l and assemb ly

performance of this corrosion inhibiting process

(SCI) is presented below.

The SCI process can be applied in either vertical or

horizontal mode, with horizontal processing being

preferred due to its simplicity and higher through

put capability.

The equipment required is typically a stand alone

unit situated, usually, directly after the Imm Ag

plating line, with the working solution delivered to

the pcb surface by traditional flood method. A

typical contact time between the pcb surface and

the SCI solution is 2.5 mins. SCI application is then

directly followed by a short rinsing stage (20-30 s),

after which the boards are thoroughly dried to

ensure good appearance and performance.

Application

Simple routine analysis procedures are required to

ensure that the process is operating at optimum

cond i t ion . The chemis t ry employed is not

aggressive to the pcb, causing, for example, no

damage to the soldermask.

The SCI material when correctly applied will result

in a chemically and thermally resistant, metal

specif ic, SAM coating being in place, covering any

exposed immersion silver and copper surfaces on

the pcb. As this coating is a SAM layer, by def inition,

is thin and because of this, the well recognized and

desired characteristics of the immersion silver

deposit are retained with the added benef its of

signif icantly improved tarnish and corrosion

resistance properties being imparted to this surface.

It is also important to note that the use of the SCI

coating does not require any changes to board

handling in the fabrication house. Assemblers will

not observe any visual differences or require any

alterations during the assembly process.

In an attempt to evaluate immersion silver and

related coatings, when exposed to a sulfur bearing

environment, MacDermid developed an in-house

accelerated tarnish test. This useful test is

described as follows: a sealed test chamber is used.

A controlled quantity of water (to induce high

humidity) and hydrogen sulf ide gas is introduced

which creates visible tarnish on the immersion

silver coating under test. Specif ically, the tarnish

chamber used is 10 cubic feet in volume and is

heated to an internal air temperature of 45 C. The

chamber contains 500mL of a sodium hydrosulf ide

solution, which when acidif ied releases hydrogen

sulf ide gas. The humidity level in the chamber,

though not strictly controlled, is typically around

80%.

O

Tarnish Resistance

Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q462

This test has been instrumental in the ongoing

development of the immersion silver process and

coating. It has proven to be sensitive enough to

allow comparison of the tarnish resistance of

immersion silver at different deposit thicknesses,

different surface pretreatments used, e.g., varying

micro-etch types, as well as aged versus new silver

deposits. The test also allows determination of the

effect of treatments applied post immersion silver

[14]. As this test was used historically to determine

small improvements in immersion silver processing,

it is now accelerated to force the immersion silver

to a very heavy level of tarnish, specif ically blue

and purple in color. Throughout our work, we

operated under the premise that demonstrating

signif icant improvement via this test would serve as

a useful hurdle for evaluating performance in other

aggressive environments. This test was employed

to a signif icant degree during the development of

the SCI coating.

The degree of protection afforded by any surface

f inish under evaluation is always compared to an

uncoated, immersion silver control set. To fully

understand the robustness of a coating, the f inish

is tested as coated and after one and two exposures

to a lead free reflow oven. We have found that

assembly heat treatments can signif icantly degrade

the ultimate corrosion resistance of a given surface

f inish. This key point is illustrated and further

clarif ied below.

As mentioned previously, there are a large number

of conventional corrosion inhibitors that can

survive the high levels of sulfur exposure similar to

those experienced within the tarnish chamber

described above. Typically, however, after lead free

solder heat excursions this tarnish resistance is

usually very signif icantly impaired (f igure 7). One

such coating is MacDermid's STI. Upon coating, the

surface is completely resistant to high humid, high

sulfur environments. The silver remains bright and

shiny. Yet, this coating is degraded after reflow and

loses its potential protection though it is better

than immersion silver unprotected. One of the

noteworthy properties of the SCI coating when

applied on to immersion silver deposits is its ability

to resist thermal degradation. This characteristic is

detailed in f igure 5C below. After two lead free

reflow excursions, the coating is uncompromised by

the sulfurous environment.

(A) (B)

(C)

Figure 7: Tarnish chamber exposure

(A) Immersion silver as coated and after 2 reflows

(B) Immersion silver with STI, as coated and after 2 reflows

(C) Tarnish chamber exposure - Immersion silver with SCI,

as coated and after 2 reflows

It was because of the great success in tarnish

resistance of this coating that we proceeded with

creep resistance testing. SCI coated samples have

been continuously tested in the "tire factory" and

"clay" chambers against immersion silver controls,

and often against OSP. Our results have indicated

that the SCI treatment does inhibit the onset of

creep corrosion, though given our test limitations;

www.hkpca.org 63

Technical Paper

it is diff icult to quantify the level of protection

provided. The photographs in f igure 8 provide some

indication of a typical result observed, but we have

observed variations in results which we cannot fully

explain at this point in our work. Though

encouraging, these results clearly illustrate the

need for the development of a standardized

industry test.

force is an alternative option to overcome the

issues encountered with OSP and other substantial

organic coatings. There are two disadvantages

associated with the use of higher spring forces

however. F irst ly, they cannot be used in

conjunction with thin circuit boards due to the

potential to cause physical damage and warping of

the boards under test. Also, higher forces promote

transfer of organic material onto the test probes

emp loyed , fu r the r i n c reas i ng ma in tenance

requirements, by way of example, f igure 9 below

details the rise in electrical resistance observed for

an OSP coating tested with the use of a 12oz spring

force due to this material transfer.

Figure 8: Creep Resistance (A) Imm Ag (B) SCI

After consistent improved performance in tarnish

and creep testing the silver f inish was thoroughly

tested in respect to all the desired and essential

qualities of an immersion silver process. The

surface f inish was put through a full product prof ile

series of performance testing both in the lab, at

third party test facilities and in commercial scale

assembly environments.

A major advantage of immersion silver over other

f inal f inishes is it superior contact functionality. It

is widely known that OSP and thick organic coatings

such as conformal coat ings are not eas i ly

electrically tested either because they can transfer

on to test probes or because they are too hard and

thick to allow the probe to penetrate to the

underlying metal surface. Some of these issues can

be overcome by the use of new probe tips styles,

such as razor or twisting probes, however, these

are generally more expensive then those used

conventionally. An increase in probe maintenance

procedures is also required as periodic cleaning

needs to be carried out. The use of a higher spring

In-Circuit Testing

Figure 9: Rise in Resistance experienced by OSP whencompared to Immersion Silver

SCI coated immersion silver does not require

unique probe styles and conventional rounded tip

probe types can be used during testing. Also,

clearly, there is no requirement for increased spring

forces to be utilized to make electrical contact with

the Imm Ag surface.

The SCI coating was specif ically designed to ensure

that this important feature of an Imm Ag coating

was retained.

Dur ing process deve lopment in i t ia l contact

resistance testing of this new protective coating

was carried out at Everett Charles test laboratories,

using a 6oz of spring force. The resistance readings

Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q464

are seen in f igure 10 below. The immersion silver

control and the SCI coated surface directly match

each other in resistance through the 1000 test

cycles carried out.

force used. The same was true for Lead Free HASL

and the SCI coated immers ion s i lver. Not

unexpectedly, the resistance readings for the OSP

coating tested illustrated one typical disadvantage

of this type of f inal f inish, inconsistency. There

were a number of instances where the resistance

measured was of the same magnitude as the

previously mentioned f inishes, however, there were

also a considerable number of measurements taken

where the resistance observed spiked well above 1

ohm. The challenges experienced with the OSP

coating and contact resistance is also observed in

the large standard deviations. These signif icant

increases in electrical resistance were not limited

only to low applied spring forces for the OSP

coating but were observed at all spring forces

applied.

Further to the above, ICT testing has been recently

been conducted on multiple sets of production SCI

coated Imm Ag panels, which conf irmed the above

and no issues or alterations to conventional ICT

procedures used for immersion silver were noted.

Initial solderability testing of SCI coated Imm Ag

surfaces was conducted in the MacDermid

Applications laboratory using a production scale

lead free reflow oven and a lead free wave solder

with foaming fluxer. A MacDermid designed

solderability test coupon was used to assess solder

spread and through-hole solderability using low

activity, "No Clean" solder paste and flux. The

solder spread was initially analyzed by simple visual

observation of manually printed surface mount

pads to characterize the wetting characteristics of

the surface. Subsequently, a varied aperture stencil

was used to evaluate the amount of spread that

resulted when a reduced amount of solder paste

was printed onto a pad. Through hole f ill was rated

based on IPC J-std 003 using a DIMM connector

Solderability

Figure 10: Contact resistance of immersion silver andsilver with corrosion inhibitor

Ano t he r mo r e de t a i l e d i n ve s t i g a t i o n wa s

s u b s e q u e n t l y c o n d u c t e d t o c o m p a r e t h e

performance of multiple f inal f inishes tested with a

variety of applied forces and probe styles. This

extensive investigation used an ICT bed of nails

f ixture specif ically manufactured for the test board

design employed. In this test, four probe sizes and

two probe styles were tested. For the scope of this

paper, only on probe style and size is reported

(f igure 11). Probes of 100mil were tested both in

the razor and chisel design. Each probe style was

tested with two probe forces.

Figure 11: ICT testing of various finishes with 100mil probes

All immersion silver resistance readings registered

well below 1 ohm irrespective of the probe style or

www.hkpca.org 65

Technical Paper

design that was tested free of a component.

All replicates in the immersion silver only and SCI

coated immersion silver displayed uniform wetting.

The varied aperture testing averaged 35% spread

for both f inishes. After four lead free reflow

excursions the spread was reduced slightly on both

f inishes to 25 to 30% spread.

Through hole solderability was also preformed on

samples with and without heat treatment. After

coating, all samples displayed 100% through-hole

f ill according to IPC J-std 003. After exposure to

four lead free reflows, the solderability was 99% or

greater on every panel.

The next level of solderabi l i ty test ing was

conducted in a production assembly facility using

automated printing and component placement. The

assembly was broken into two segments, a portion

of the panels were exposed to one lead free reflow

followed by print, component placement and reflow,

resulting in two lead free reflow exposures. The

second set was processed in an identical fashion

but was fol lowed with manual through-hole

component stuff ing and a lead free wave solder.

Surface wetting was analyzed visually. Solderability

under components and in the through-holes was

analyzed via x-ray inspection using IPC J-std 003

criteria. Again, for this testing a low activity "No

Clean" SAC 305 solder paste was used in

combination with a low activity no clean flux. All

testing was performed in an air environment (no

nitrogen blankets).

Illustrated below are the results obtained for

immersion silver only, SCI coated immersion silver

and high temperature OSP tested under the above

mentioned conditions. Visual observation of the

solder spread was carried out using a FET feature.

A grid of 5 by 5 round solder areas were printed in

the FET square, no component was placed. Analysis

was done on samples that were printed on the f irst

and second reflow cycle to observe the difference in

spread as a result of heat exposure. The spread was

rated based on the level at which the 25 printed

areas flowed together and over the entire FET

square.

For immersion silver and immersion silver with the

SCI coating, all the printed areas connected and the

solder flowed to the edges of the square, see f igure

12 (A and B) below. This was consistent for one and

two lead free reflow exposures.

The high temperature OSP resulted in minimal

connection of the printed areas, see f igure 12 (C

and D) below. After one reflow exposure then print

and reflow, there was no connection of the solder.

The solder reflowed but remained in its printed

areas.

Wave soldering was also conducted in two sets.

One set was exposed to one lead free reflow before

wave soldering and the other was exposed to two

Figure 12: Immersion Silver with SCI solder flow after(A) print and lead free reflow, (B) 2 reflows

Figure 12: (A) OSP solder flow after print and lead free reflow(B) 2 reflows

Technical Paper

Journal of the HKPCA / Issue No. 38 / 2010/ Q466

lead free reflow cycles prior to the wave. For

through-hole solderability, each panel processed

contained four through-hole components with 16

leads per component for a total of 64 holes

analyzed per panel. Figure 13 shows the percent

hole f ill over the all components. The blue bar

represents one reflow exposure and the set after

two reflows is represented by the purple bar in the

same f igure. The hole-f ill defects were observed by

x-ray which showed the level of f ill that was

reached in each barrel. Notably, any hole defect

observed for the immersion silver or SCI coated

Imm Ag surface still resulted in hole f ill greater

than 80%, whereas OSP holes with defects were

found to be typically as low as 40% f illed.

and referenced in this paper gives tangible

evidence to gaps which currently exist between

industry accepted corrosion tests and "real world"

results seen in a relatively diverse group of end use

environments. From this, it is clear that these gaps

in test must be closed for eff icient progress to be

made. The work described here notes specif ic

va r iab les and fac to rs in tes t ing (such as

condensation and airborne particulate influences)

which appear to encourage creep corrosion growth.

It is proposed that these factors are likely part of a

f inal test design appropriate for this industry issue.

Further, utilizing test methods developed and

described above, this work describes advancement

in the corrosion resistance of an immersion silver

f inal PCB f inish. The SCI f inish exhibits improved

resistance to sulfur and sulf ide based harsh

env i ronments , comb ined wi th re ten t ion o f

performance after multiple Pb-free assembly heat

excurs ions . Ev idenced by commerc ia l - sca le

a s semb l y wo rk and suppo r t i ng func t i ona l

performance testing, this coating retains all

desirable performance attributes of conventional

immersion silver.

[1] Schuller, R., "Creep Corrosion on Lead Free

P r i n t ed C i ru i t Boa rds i n H igh Su l fu r

Environments" SMTA International Orlando,

Oct. 2007

[2] Sandia National Laboratories, "The Effects of

Varying Humidity on Copper Sulf ide Film

Formation." Sand Report, Feb. 2004

[3] Chen, Xu., "Creep Corrosion of PWB Final

F i n i shes : I t s Cause and Preven t i on" ,

IPC/APEX April 2009.

REFERENCESFigure 13: Through-Hole Solderability

It is clear from the above extensive testing that the

cons istent so lderabi l i ty performance of the

immersion silver f inal f inish is fully retained after

SCI coating. This has also been reconf irmed by

further detailed testing by a variety of OEM's on

boards from a full production environment.

This paper calls attention to issues surrounding the

performance of PCB f inal f inishes in harsh end use

environments. The improved affordability and

ut i l i ty o f e lec t ron ic goods, combined wi th

increasing industrialization in emerging markets

has driven this phenomenon. The work described

CONCLUSION

[4] A b b o t t , W., " T h e D e v e l o p m e n t a n d

Performance Characteristics of Mixed Flowing

Gas Environment", IEEE Trans. Components,

Hybrids, manufacturing Technology, Vol.

CHMT-11:1, 1988, p. 22-35

[5] ASTM International, "Standard Guide for

Mixed Flowing Gas (MFG) Tests for Electrical

Contacts", ASTM B 845-897, June 10 , 2003.

[6] ISA-S71.04-1985, "Environmental Conditions

for Process Measurement and Contro l

S y s t e m s : A i r b o r n e C o n t a m i n a n t s " ,

Instrument Society of America, 1985.

[7] Helen Holder, HP presentation Metal Finishes

Data Acquisition Task Group (IPC 3-11g)

committee conference calls.

[8] Benchmarking Urban Air Quality Management

and Practice in Major and Mega Cities of Asia,

APMA, 2002.

[9] Improving Air Quality in Asian Developing

Countries, Chinese NRI Activities, Phase 1

Final Report , As ian Regional Research

Programme on Environmental Technology

(ARRPET), May 2004.

[10] www.ipc.org Task Group 3-11 g

[11] Toscano, L., Long, E., Swanson, J., "Creep

Corrosion on PCB Surfaces: Improvements of

Predictive Test Methods and Developments

Regarding Prevention Techniques", SMTA

International Orlando, Oct. 2007

[12] Toscano, L., Long, E., "Creeping Corrosion of

PWB Surfaces in Harsh Sulfur containing

Environments", SMTA International Orlando,

Oct. 2008.

th

[13] Bratin, P., Pavlov, M., , 1999 May p.30-

37.

[14] Toscano, L ., Cu l l en , D., "The S tudy,

Measurement, and Prevention of Tarnish on

Immersion Silver Board Finishes", IPC July 03.

PC Fab

www.hkpca.org 67

Technical Paper