Electronics Surface Finish OverviewSMTA Upper Midwest Expo

Robyn J HansonMacDermidJune 25, 2015

Agenda

• Overview of Surface Finish Needs� Market drivers

• Current Surface Finishes� Detailing the most popular

• Applications of each finish

• Process steps

• Past issues/beliefs

• Remedies

• Pros and cons

• Summary

p.2

One Size Fits all?

• Which surface finish is “the best”?� Final Finishes include: Immersion Tin, Immersion

Silver, OSP, ENIG, ENEPIG, HASL - lead and lead-free, Palladium over Copper and Electrolytic Ni/Au to mention a few

• People prefer one size fits all� Unfortunately, the truth is, there is no one final finish

to fulfill all the requirements in the industry, so - 'it depends on your needs’

p.3

Considerations for Surface Finish Choice • Lead or lead free assembly• Cost• End environment• Shelf life• Volume and Throughput• Fine pitch components • RF applications

� High Frequency

• Probeability• Thermal resistance• Shock and drop resistance

p.4

Overview: for the OEM

• Performance expectations of PCB surface finishes aregreater than just solderability preservation� Historically, surface finishes were designed solely to protect

copper from oxidation prior to the soldering of components

• Today’s expectations include: superior solderability,contact performance, wire bondability, corrosion and thermally resistant, an extended end use life and mind you, all at a low cost

p.5

Overview: for the OEM

• Designs have changed� Lines and spaces are reduced, solder types and flux

chemistries are different due to no lead demands, there are ahigher percentage of no clean assemblies, number of assembly cycles has increased, the product needs to carry high frequency signals so the surface must be flat

� Significant amount metal area left exposed in the end-use applications that needs to be protected from the environment

p.6

Overview: for the Fabricator

• No choice� in many cases, the surface finish is dictated by the OEM, with

the prints calling out the required surface finish

• in some instances, fast turn prototyping, there may be room forthe fabricator to choose

• In many cases, the prints will offer more than one finish, which is left to the fabricator

• The fabricator’s expectations: high throughput, ease of analysis, consistency, low defect rate, visual inspection, conforms to all environmental and government regulations, extended bath life, lowcost, low water consumption and waste costs

p.7

Growth Segments in the Electronics Industry

Handheld DevicesSmartphones, tablets

Infrastructuresupport high-function portables

Automotiveunit growth and increasing content

Automotive

Handheld Devices Infrastructure

p.8

Surface Finish Market Share - Worldwide

p.9

HASL: Hot Air Solder Leveling

• The oldest surface finish• There are Pb and Pb-free versions• Leaded HASL currently in limited use due to

RoHS and WEEE initiatives• Currently Exempt

� Industrial Vehicles

� Military

� Aerospace and Defense

� High Performance Electronics

• Leaded versions harder to source

p.10

HASL: Process Steps

• Clean• Microetch• Flux• Solder dip• Air knife leveling

� The shape of the solder will be irregular - not flat

• Post clean

p.11

HASL: Pros and Cons• Advantages

� Low Cost

� Re-workable

� Excellent solder wetting

� Long shelf life

• Disadvantages� Line maintenance

� Board warpage• Overall aggressive on panel

� Not suited for fine pitch• Components do not lie flat

� Copper dissolution

� Paste misprints – Assembly

� Lead free• Wetting problems• Soldering irons may not get

hot enough• Reduced PTH reliability

p.12

Issues at the Fabricator Level

• Copper cleanliness� Critical to the application of the coating

• For fine pitch components, HASL has difficultlybreaking the surface tension caused by the soldermask� Results in skip pads – creating an open

• For tight line/spacing� The excess solder may not be completely removed,

so the solder may bridge – creating a short

p.13

Issues at the OEM Level

• HASL is blown from thePCB surface to remove excess, this creates nonuniform coverage

– Very difficult to place components

• The hot temperatures of lead-free HASL can cause warpage and soldermask embrittlement� Difficult component placement

� Access to copper for corrosion

� Reduced PTH reliability

p.14

OSP: Organic Solderability Preservative

• Highest volume surface finish� Applications range from low end

to high frequency server boards

� Also used in selective finishing

• Latest versions are copper selective and morethermally resistant

• OSP is applied through chemical adsorption on the copper surface – no metal to metal displacement

p.15

OSP: Process• Process Steps

� Cleaner

� Rinse

� Microetch

� Rinse

� Neutralizer

� Rinse

� OSP

� Rinse and dry

p.16

OSP: Improvements• The copper was replaced with a more thermally

stable inorganic material� It can withstand higher temperatures and multiple

reflows without degrading

� It lessens oxygen permeation and the organic hardening of the OSP, which leads to oxidation of the copper

� In selective finishing, i.e. when ENIG is applied as the surface finish, OSPmay be applied to the BGA areas dueto concerns over black pad

• will not adhere to or stain any gold surfaces, no need to plasma clean

p.17

OSP: Pros and Cons

• Advantages� Easy process

• Short, automated, easy to analyze

� Very inexpensive

� Reworkable

• Disadvantages� Line maintenance

• Crystals collect on rollers

� Difficult to inspect, handle

� Cannot measure thickness

� Difficult to probe• No electrical test

• No wire bonding

� Limited shelf life in assembly

� Poor solderability after reflow

p.18

Issues at the Fabricator Level• Copper cleanliness

� Critical to the adsorption of the coating

• Line cleanliness� During shutdown, material can dry/solidify on rollers which will

cause defects

• pH effect on coating thickness� Increasing the pH of the OSP bath increases the coating

thickness, but bath becomes unstable

• Not easy to inspect� Leaves a sheen on the copper surface, but could require a

trained eye to understand uniformity

• Thickness measurement is assumed� Based on a thickness panel and assumed coating density

p.19

Issues at the Assembly Level

• Not thermally resistant� Coating hardens with reflow exposure

• Becomes more difficult to solder

� Probing issues• Material transfers to probe tip resulting in false readings

• Will require more frequent probe maintenance

• Could require a special probe style

� Higher OSP thicknesses detrimental to solder paste flow and hole fill

p.20

Solder Spread

• Solder stays where it is printed• When comparing to a metallic surface finish,

difference is dramatic• No spread leaves more exposed metal

OSPImmersion Tin

p.21

Immersion Tin

• Applications are predominatelyAutomotive, US Military andAerospace

• Excellent for press fitapplications� Large back panels

• All contain anti whiskering additives,but tin whisker elimination is not guaranteed

p.22

Immersion Tin Process

• Process Steps:� Cleaner

� Rinse

� Microetch

� Rinse

� Predip

� Immersion Tin

� Rinse

� Post Dip

� Rinse and dry

p.23

Immersion Tin: Pros and Cons

• Advantages� Process ease

� Well suited for pin insertion applications

� Reworkable

� Low cost

� Flat, fine pitch

� Solder and Tin dissolve quickly in the solder at assembly, making a good copper/tin solder joint

• Disadvantages� Aggressive on

soldermask

� Solderability quickly degrades with oneheat exposure

� Whisker potential

� Quick intermetallic• Reduced shelf life

p.24

Issues at the Fabricator Level• Copper cleanliness

• Maintenance and add backs are critical tomaintain anti whiskering agents

• Line cleanliness� Over heating bath/introduction of

oxygen will result in tin oxide (Sn4+)

� During shutdown, material candry/solidify on rollers and cause defects

• Poor control of plating rate can result in adhesion issues

• Pure tin thickness measurement is critical� Older XRFs measure ‘all’ tin and do not give a pure tin reading

p.25

Tin Copper Intermetallic

• Copper and tin naturally migrate to form an intermetallic, so migration begins immediately after plating, creating the intermetallic layer (IMC)

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• A minimum of 16 microinches of ‘pure tin’ is necessary to protect the copper and intermetallic against corrosion

• Once the intermetallic formation eliminates all ‘pure tin’ on the surface, the solderability is deteriorated

• It is important to target the OEM and chemical supplier pure tin thickness requirements

p.26

Issues at the Assembly Level

• Pure tin thickness is lost to the copper intermetallic with time and temperature� Loss of pure tin will degrade solder performance

• The first reflow exposure will dramaticallyreduce the pure tin thickness� Chemical formulations are designed to reduce this from

occurring

• Deposit stress results in tin whiskers� This is a naturally occurring characteristic of tin in direct

contact with copper

p.27

Tin Whiskers

• The driving force for the formation oftin whiskers is the strong bond createdbetween the tin and copper and theformation of the intermetallic phasecreating a mechanical stress in the deposit, which is released as a whisker

• The formation of the IMC phase is faster at higher temperatures, but at higher temperatures more mechanical stress in the deposit is released� Tin whiskers may be reduced by heating the substrate to

120 C for 90 minutes, but this is not practical for PCBs• Shelf life will be reduced, due to loss of pure tin

p.28

Combating Tin Whiskers• Heat treatments, such as reflow, release stress in the

tin deposit reducing the propensity for whisker growth

• Formation of tin whiskers on immersion tin is reliablysuppressed by doping the tin deposit with an anti-whisker additive (AWA) which alters the grain structure to create defined structures with uniform distribution resulting in less stressIncorrect AWA in the bath Optimized AWA in the bath

p.29

Immersion Silver

• Greatest conductivity of allthe surface finishes� Well suited for all high

frequency applications• Flat, reduces signal loss

• Applications range from lowend to high reliability product

• Topcoats have been formulated to overcome tarnish and corrosion issues in aggressiveenvironments

p.30

Immersion Silver Process

• Process Steps:� Cleaner

� Rinse

� Microetch

� Rinse

� Predip

� Immersion Silver

� Rinse and dry

p.31

Immersion Silver: Pros and Cons

• Advantages� Flat/fine pitch

� Low cost

� Excellent wetting

� Good surface contact

� High throughput

� Easy process

� Excellent solderability• Not degraded by

assembly conditions

• Disadvantages� Handling

requirements• Easily scratched

� Migration concerns• Creep

� Tarnish and corrosion resistance

� Sliding connector limitations

p.32

Issues at the Fabricator Level

• Copper surface cleanliness� Critical to achieving a successful silver finish

� No soldermask residues

• Heavy = skipping of the silver coating• Fine = may go unnoticed, manifests as a tarnish of pad edges after reflow

� Soldermask interface attack

• Fine line connected to a large pador through hole area

� Proper silver thickness

• Thickness measurements need to be measured with thin foil standards on the XRF

• Silver thickness below specification will lead to premature tarnish

� Tarnish

• Proper processing and handling eliminates the majority of tarnish instances

p.33

Issues at the OEM Level

• Microvoiding� Small voids occurring at the IMC layer of the solder

joint

� Could cause solder joint fracture

p.34

Microvoid Research

• This defect manifests itself “preferentially” on soldermask defined pads which are more difficult to develop properly

• Extensive microvoid research shows thatmicrovoids can be directly correlated tosoldermask residues and highly active, rough copper surface topography when all otherImmersion silver conditions are run properly� New cleaners remove soldermask residues

� New microetches minimize copper surface area, yet maintain surface roughness for adhesion

p.35

Creep Corrosion

Creep corrosion: a metal migration of copperand sulfur as a result of high humid, highsulfur bearing environments

• PCB’s are everywhere� Marked increase in pollutants in developing

countries

� Placed in diverse industrial environments• Tire factory, paper mill, automotive design studio

• Even our controlled office environments are not asclean as we think!

p.36

Creep CorrosionCreep Corrosion can occur with almost any final finish – but is more prevalent on Immersion Silver

OSP

Immersion Silver

p.37

Topcoats for Immersion Silver

• Topcoats are used for products with lifeexpectancy over 15 years in aggressive fieldenvironments where the silver surface is leftunsoldered or in a no-sealed build� Specific topcoats to mitigate creep corrosion

� Specific topcoats to mitigate metal peeling and tarnish

p.38

ENIG

• Highest revenue surface finish� Applications associated with high reliability

� Used in the flex market

• High volume runner• New deposit thickness specifications

� IPC thickness specification is under revision• High cost of gold

• Hypercorrosion/black pad

p.39

ENIG Process• Process steps

� Cleaner

� Rinse

� Microetch

� Rinse

� Pre dip

� Activator

� Rinse

� Post dip

� Electroless Nickel

� Rinse

� Immersion Gold

� Rinse

p.40

ENIG: Pros and Cons

• Advantages� High corrosion

resistance due to nickel barrier

� Excellent solderability

� Contact resistance

� Al wire bondable

� No degradation between cycle reflow cycles, so it can be held mid-assembly for extended times

• Disadvantages� Many chemical steps

• Difficult processing

• Potential for extraneous plating, plating in the NPTH

� Expensive

� High bath temperatures leach soldermask and substrate materials

� Signal loss (RF)

p.41

Issues at the Fabricator/OEM

• Proper plating conditions and control over the entire process is critical to performance� Proper chemical addbacks

� Numerous chemical analyses – startup and during plating

• Layer thickness� Low EN thickness will result in poor corrosion and thermal

resistance in end use

� Gold thickness

• Low: less resistant to thermal conditioning in assembly

• High: promote more EN corrosion – black pad

• Loading factor� Too much or not enough metal area in the plating bath will affect

the plating performance

p.42

ENIG: Past Issues

• Plating in the NPTH� Both issues resolved with new

cleaners and improved activators

• Extraneous plating

p.43

ENIG: Black Pad

What is Black Pad/Black Line Nickel/Hypercorrosion?

• Low level defect, not typically detected before component assembly� Excessive corrosion of the electroless

nickel/phosphorous deposit• As the immersion gold plates, nickel

is removed from the surface of theplated EN, leaving phosphorous rich areas on the surface, observed as blackon this image

• When a solder joint is created with thissurface phosphorous, instead of the EN, it creates a weak joint which can fracture

� Results in improperly formed solder joints

� Detected as an open at electrical test after assembly

p.44

ENIG: Black Pad

Mitigation:• New electroless nickel chemistry formulations

� Higher, consistent phosphorous content uniformly distributed in the plated deposit makes the nickel more corrosion resistant

� Chemistries contain less sulfur, so it has a reduced tendency to co-deposit making the nickel more corrosion resistant

• New immersion gold chemistries� Additive allows the gold bath to operate at lower gold

concentration without sacrificing performance, so the gold solution is less aggressive to the nickel deposit

p.45

Measurement of Corrosion – IPC 4552

p.46

Corrosion resistance of the electroless nickel deposit

ENEPIG

• The gold wire bondablefinish� Also Aluminum wire bonding

• Very low volume associated specifically to PCB� Applications include Medical and US Military

• Has been a topic of interest for many years• OEMs and fabricators are becoming more

interested in this surface finish

p.47

ENEPIG ProcessSame flow as ENIG, with the added step of electroless palladium between the nickel and gold

� Cleaner� Rinse� Microetch� Rinse� Pre dip� Activator� Rinse� Post dip� Electroless Nickel� Rinse� Electroless Palladium� Immersion Gold

p.48

ENEPIG: Pros and Cons

• Advantages� High Corrosion

resistance

� Excellent solderability

� Gold wire bondable• ENEPIG requires 4µ” Pd

and 1.2µ” Au, while electrolytic Ni/Au requires >30µ” Au

� Mitigation of black pad• Pd prevents ImAu from

corroding the nickel

• Disadvantages� Many chemical steps

• Difficult processing

• Potential for extraneous plating

� Very expensive

� Long process

� Limited availability• We see this growing

p.49

Issues at the Fabricator Level

• Same considerations for running ENIG with added complexity of electroless palladium step

• Low production can mean a lot of down time� Chemistry needs to be well maintained and

monitored• When baths sit idle at temperature, the bath components

break down and performance is lost

• Thickness measurements are more difficult to determine with 3 layers of plating� XRFs require calibrations which is difficult using a

standard with 3 layers of plating

p.50

Issues at the OEM Level

• Too thick Palladium reduces the solderability performance� Slower to wet

� Potentially Palladium rich areas in the solder joint• Palladium does not readily solubilize into the joint like silver

or gold

p.51

Thickness Measurements – IPC 4556

p.52

IPC 4556 states a thickness of 2-12 �#�Pd and min of 1.2 µ” Au

•IPC looking to input a gold thickness maximum and place a range of 1.6 to 3.2 or 3.4 microinches – under review.

Old proportional counter XRF units will not be able to measure these specifications and the fabs will be required to use the specified standards for calibration

Summary: Surface Finishes by Sector• Data/telecom

� Silver, OSP, ENIG

• Automotive� Silver, OSP, ImSn

• High End Consumer� ENIG, Silver, OSP

• Low End Consumer� HASL, OSP

• Aerospace, Defense and High Performance Electronics� HASL, ImSn, ENIG, ENEPIG

• Medical� ENIG, ENEPIG, Silver

p.53

Cycle Comparison of Surface Finishes

p.54

Cycle Times

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SummaryThere is no one surface finish that is suitable for all

applications and environments in the industry

Considerations for Surface Finish Choice Include:• Lead or lead free assembly• Cost• Volume and throughput• End environment• Shelf life• Fine pitch components • RF applications

• High frequency

• Probeability• Thermal resistance• Shock and drop resistance

p.55

p.56

THANK YOU FOR YOUR TIME!

Robyn J Hanson

MacDermid

June 25, 2015