|Ask the Experts|
January 5, 2021
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?
|Expert Panel Responses|
There's a couple things here.
First, exposed surface finish is not a defect condition according to IPC-A-610E. Section 5.2.1 states that an acceptable connection can have "Exposed surface finishes that are not part of the required solder fillet area."
As long as the solder connection has the correct amount of hole fill (75% hole fill in most cases), solder wetting around the wire and barrel of the hole on the solder source side (270 degrees class 2, and 330 degrees class 3), and the proper amount of wetting on the solder destination side (180 degrees class 2, and 270 degrees class 3) you do not need to touch the connection again.
Reheating a connection multiple times when not required could heat damage the connection or increase the thickness of the brittle intermetallics among other things.
The other question I have is, are you removing the gold first? If not, you might be setting yourself up for failures due to gold embrittlement.
Manager of Assembly Technology
Wave soldering requires careful optimization of several variables to insure adequate hole-fill.Is the flux you are using active enough, is the contact time and immersion 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 flux manufacturer?Excess preheat de-activates the flux and insufficient heat does not vaporize the solvents, making the solder contact more for vaporizing the solvent and less hole-fill.
Are you also applying enough flux?
The last item is solderability. Is the base metal under the gold solderable enough for the flux you are using?
Senior Market Development Engineer
It's almost always harder to fill open holes than holes with leads in them. The presence of a lead in a hole helps with heat transfer, and creates larger capillary force (the force that helps solder wick up the barrel).
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 the applicable standards for the class of the assembly, as defined by you (or your customer).
Any additional soldering time or touch-up steps beyond what's required to meet the standards will increase the heating experienced by the through-hole and may actually be detrimental to the ultimate reliability of the assembly.
It's not uncommon for this effect, and probably more common on all solder joints... but due to dissimilar colors, you notice it more on the Au plated annular ring.
To minimize the effect can be a function of the solder mask, and the cleanliness of the PCB prior to soldering.
If you have control of the artwork, you might consider the annular ring to be more proportional to the 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 you can determine what is a value add to the inspection criteria.
Capital Equipment Operations Manager
Specialty Coating Systems
The first question is how many layers of interconnect are there in these holes typically? 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 you seeing any other through hole defects or just on these jumper holes?
Is the assembly 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 soldering process?
S T and S Testing and Analysis
There's a variety of causes for the solder not to fully wet to pads & hole walls, and none of them are good. And it gets worse: Non wetting may indicate overall poor bonding of the solder joint for the areas that actually appear wetted.
Best begin with a bare PCB. I would evaluate for board cleanliness and correct gold-type. Then confirm that the jumpers are clean. Proceed to assembly and validate correct time & temp. at wave, and don't forget wave purity. Somewhere along the way I feel the root-cause will present itself.
If a bare board is not available, consider submitting a worse-case assembly to a service bureau specializing in post-assembly contamination detection & analysis. That science and service has greatly advanced in the last 5-years.
I'm really interested in the jumpers. PCB contamination is often broad, not selective. Ditto for gold plating, presuming the entire PCB uses gold as a final finish.
Circuit Connect, Inc.
So many answers are possible be it leaded or Lead free soldering BUT the one that may best fit is lack of Nickel in the plating and this gives good bottom side wetting but very poor capillary action.
Technical Sales Manager
BLT Circuit Services Ltd
Have the board fabricated with solder coating in the holes... and avoid removing gold from the holes.
SME - PWB Technologies
Gold is the best finish to use to stop oxidization of the boards but is not always the easiest to put on. This is especially true in through hole plating. You often find that the gold can be very thin at the shoulder of the hole or at some point in the barrel, this can lead to poor penetration or incomplete wetting on the topside.
The difficulty is that as the issue is the thickness of the gold and the potential for oxidization of the nickel surface beneath it you cannot really inspect before processing to see if it is OK. You can X-section the failed areas to determine the cause of the defect but that does not solve the problem.
You could experiment with other surface finishes if this is allowed to see if they give better results, tin and silver can sometimes work better but have a shorter shelf life, the advantage with these finishes is that if there are only very small areas not soldered then they do not show up as easily as gold. the disadvantage is that if there are only small areas that are not covered then they do not show up as easily as gold.
Global Product Champion
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 u inches of gold, not enough to cause any issues.
On the other hand, if insufficient gold is used to seal the nickel under plate 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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