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The Knowledge and Know-how Connection
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June 18, 2013
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Can Immersion Gold PCBs Oxidize?
I have immersion gold PCBs that have developed oxidation. Is there a specific material/process to recover these boards? There seems to be a disconnect here, can PCBs with immersion gold oxidize?
Board Talk is presented by ITM ConsultingVisit ITM Consulting to learn more.Phil Zarrow, ITM ConsultingWith over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes. Jim Hall, ITM ConsultingA Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.
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Views: 3135
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Immersion Gold over Ni (ENIG) is by nature a porous and extremely thin finish. Unfortunately ENIG is the most common surface finish in the world (for no good technical reason) - leading to an incredible amount of reliability problems.
ENIG is no good for solder joint reliability in portable electronics because of formation of the brittle Ni-Sn InterMetallic Compound. It therefore have been banned by most of the serious mobile phone brands for BGA/CSP footprints the last 5 - 10 years. OSP is significantly cheaper and gives much better solder joint strength.
ENIG is furthermore a very unreliable surface finish for electromechanical contacts or keypads: the porosity and low scratch resistance of the typical only 25-60 nm thin gold leads to exposed Ni that easily corrodes - resulting in contact reliability issues.
If you want to have a PCB contact pad you can rely on it have to be thick electrolytic gold > 0.8um or Carbon surface finish. Take a look at my technical paper: New trends for PWB surface finishes in mobile phone applications. You can download it from here: http://cwngroup.dk/about/technical-papers/
Claus W. Nielsen, E-Consult, Denmark
You are correct gold does not oxidize, but immersion gold is porous no matter how much you deposit. A thinner layer of gold does not necessarily mean less protection of the underlying metal, but it does mean that the underlying metal experienced less corrosion during the immersion process.
James Economus, EMS Analytical Labs, Inc.
Even non-porous layers will get problems if there is no barrier layer between the gold and copper. There should be a well sealed nickel layer (Electroless Nickel-Gold) between the gold and copper. If the nickel is porous, copper atoms migrate through the gold to the surface where they oxidize to copper oxide.
Pieter Hoeben, Hoeben Electronics
Our experience shows that ENIG with gold plating thickness in the low 2- 2.5 micron range can cause some nickel oxidation, especially if the gold is on the porous side. Typically ENIG boards require only a mild flux but to solder these boards just use a stronger flux. No other rework can really be done.
Bob Gardiner, Tekelec
Any gold immersion coated board should be placed in a metallized ESD bag and sealed well. Then store in a nitrogen purged dry box. It will still only last up to 6 months before the gold becomes duller due to the diffusion of the substrate. The more expensive and much thicker auto-catalytic electroless gold process will be a better approach.
Jerry Fang, Northrop Grumman
Pure gold does not oxidize. Co-deposited metals can. However, many problems are caused by the porosity of the gold coating, allowing the substrate metals (Ni or Cu) to oxidize and causing solderability problems.
Brian Ellis, Cyprus
Immersion gold on copper is usually thin (approximately 5 microinches) and has a degree of porosity which will lead to oxidation. In addition, if the coating is just gold over copper, diffusion will continue until all the gold has gone. Even the current ENIG (electroless nickel/immersion gold) process has a limited life for solderability because the immersion gold is thin.
Edward Cuneo, Boeing
I agree with Edward. We have been using ENIG boards for 5 years and have never had an oxidation problem. Boards are not stored more than a year or so and are in a humidity controlled environment. If the boards are ENIG cleaning and reflow of the boards may drive off the oxidation.
Theresa Russell, Doble