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September 12, 2018

Gold Plated Hole Defects

Recently we began wave soldering assemblies that have gold plated holes that are used for jumper wires. After wave soldering, there are often portions of the gold plated holes and pads that still have exposed gold. I'm not sure if it's necessary, but we touch up these gold areas by hand soldering. What is the best way to insure we get full solder coverage on these gold plated holes and pads?

C.W.

Expert Panel Responses

There's acouple things here. First, exposedsurface finish is not a defect condition according to IPC-A-610E. Section 5.2.1states that an acceptable connection can have "Exposed surface finishes thatare not part of the required solder fillet area." As long as thesolder connection has the correct amount of hole fill (75% hole fill in mostcases), solder wetting around the wire and barrel of the hole on the soldersource side (270 degrees class 2, and 330 degrees class 3), and the properamount of wetting on the solder destination side (180 degrees class 2, and 270degrees class 3) you do not need to touch the connection again. Reheating aconnection multiple times when not required could heat damage the connection orincrease the thickness of the brittle intermetallics among other things. The other question Ihave is, are you removing the gold first? If not, you might be setting yourselfup for failures due to gold embrittlement.

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Kris Roberson
Manager of Assembly Technology
IPC
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

Wave soldering requires careful optimization of severalvariables to insure adequate hole-fill.Is the flux you are using active enough, is the contact time andimmersion depth adequate or should they be increased? The preheat also has to be sufficient at the areas of concern. Is it according to the prescribed requirement of the fluxmanufacturer?Excess preheat de-activates the flux and insufficient heat doesnot vaporize the solvents, making the solder contact more for vaporizing thesolvent and less hole-fill. Are you also applying enough flux? The last item issolderability. Is the base metal under the gold solderable enough for the fluxyou are using?

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Peter Biocca
Senior Market Development Engineer
Kester
Mr. Biocca was a chemist with many years experience in soldering technologies. He presented around the world in matters relating to process optimization and assembly. He was the author of many technical papers delivered globally. Mr. Biocca was a respected mentor in the electronics industry. He passed away in November, 2014.

It's almost always harder to fill open holes than holes withleads in them. The presence of a lead in a hole helps with heat transfer, andcreates larger capillary force (the force that helps solder wick up thebarrel). There is no reason,from a reliability standpoint, that you must solder the entire hole and coverall of the gold. When you install the jumper, the solder joint must meet theapplicable standards for the class of the assembly, as defined by you (or yourcustomer). Any additional soldering time or touch-up steps beyond what'srequired to meet the standards will increase the heating experienced by thethrough-hole and may actually be detrimental to the ultimate reliability of theassembly.

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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

It's not uncommon forthis effect, and probably more common on all solder joints... but due todissimilar colors, you notice it more on the Au plated annular ring. To minimize theeffect can be a function of the solder mask, and the cleanliness of the PCBprior to soldering. If you have controlof the artwork, you might consider the annular ring to be more proportional tothe size of the lead and hole. The size of the hole to lead ratio,coupled with solder tension can contribute to the solder fillet shape. Minimize the lead/hole ratio and better match the annular ring and youcan determine what is a value add to the inspection criteria.

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Rodney Miller
Capital Equipment Operations Manager
Specialty Coating Systems
Rodney is currently Operations manager at SCS coatings, Global Leader in Parylene and Liquid Coating equipment. Rodney applies his BS in Computer Integrated Manufacturing from Purdue University, along with 20+ years of Electronic manufacturing and Equipment Assembly, to direct the Equipment business at SCS Coatings. "We provide unique, value added coating equipment solutions for our customers". Including conformal, spin and Parylene coating expertise.

Thefirst question is how many layers of interconnect are there in these holestypically? When you say exposed gold, do you mean incomplete hole fill and nontop side annular ring wetting? Please confirm that the gold is ENIG. Are youseeing any other through hole defects or just on these jumper holes? Is theassembly in a pallet or held by the edge going through the wave solder process.What is the top side temperature of the board during the wave solderingprocess?

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Gerard O'Brien
President
S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.

There's a variety ofcauses for the solder not to fully wet to pads & hole walls, and none ofthem are good. And it gets worse: Non wetting may indicate overall poor bondingof the solder joint for the areas that actually appear wetted. Possibilities
  • The pad/hole wallsurface is contaminated from the PCB Fabricator.
  • The dwell timeand/or temperature at wave are inadequate.
  • The jumper wiresthemselves are not clean an are introducing contamination.
  • The gold is the wrongtype (e.g., electro-gold versus ENIG).
Getting Started Best begin with abare PCB. I would evaluate for board cleanliness and correctgold-type. Then confirm that the jumpers are clean. Proceed toassembly and validate correct time & temp. at wave, and don't forget wavepurity. Somewhere along the way I feel the root-cause will present itself. Contamination Forensics If a bare board is not available, considersubmitting a worse-case assembly to a service bureau specializing inpost-assembly contamination detection & analysis. That science and servicehas greatly advanced in the last 5-years. Best-Guess? I'm really interested inthe jumpers. PCB contamination is often broad, not selective. Dittofor gold plating, presuming the entire PCB uses gold as a final finish.

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Robert "Bob" Lazzara
President
Circuit Connect, Inc.
Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.

So many answers are possible be it leaded orLead free soldering BUT the one that may best fit is lack of Nickel in theplating and this gives good bottom side wetting but very poor capillary action.

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Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Havethe board fabricated with solder coating in the holes.. and avoid removing goldfrom the holes.

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Mahendra Gandhi
SME - PWB Technologies
Northrop Grumman
Mahendra Gandhi has been working in interconnect industry since 1972.

Goldis the best finish to use to stop oxidization of the boards but is not alwaysthe easiest to put on. This is especially true in through hole plating. You oftenfind that the gold can be very thin at the shoulder of the hole or at somepoint in the barrel, this can lead to poor penetration or incomplete wetting onthe topside. the difficulty is that as the issue is the thickness of the goldand the potential for oxidization of the nickel surface beneath it you cannotreally inspect before processing to see if it is OK. You can X-section thefailed areas to determine the cause of the defect but that does not solve theproblem. Youcould experiment with other surface finishes if this is allowed to see if theygive better results, tin and silver can sometimes work better but have ashorter shelf life, the advantage with these finishes is that if there are onlyvery small areas not soldered then they do not show up as easily as gold. thedisadvantage is that if there are only small areas that are not covered thenthey do not show up as easily as gold.

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Richard Boyle
Global Product Champion
Henkel Electronics
Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.

Mr. Roberson is correct. I suspect you are using lead-free solder which does not wet out to the edges of the pad and as a result there may be small amounts of gold still showing which is not a defect condition. As far as the danger of embrittlement, if the PWB plating is ENIG and meets the criteria of IPC 4552, there is little probability you will have any issues with that, as ENIG has less than 5 uinches of gold, not enough to cause any issues. On the other hand, if insufficient gold is used to seal the nickel underplate from oxidation (which is what you are really soldering to) then the oxidation would also cause poor wetting. However, I doubt that this is happening, because then you would not see any gold in the non-wetted areas.

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Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.