Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

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Gold versus HASL finish

#33204

Gold versus HASL finish | 15 March, 2005

We are looking for some feedback on this situation. Please let us know if you have any ideas. Thanks in advance.

I have been doing some studies on defects and such with our PCB's in prep for lead free and I've come to a conclusion about our process. We currently use boards with a 63/37 Sn/Pb finish and also use boards with a nickel/immersion gold finish. We have fantastic results with the Sn/Pb finished boards, but seem to have some issues with the nickel/immersion gold board finish boards. In looking at the defects the only thing that truly stands out is the appearance of the joints. For the most point the joints meet standards but appear to be really starved for solder and/or very dull. It looks to me as if the solder flows to the gold finish and then has to fight its way back up the part. If there is any type of oxidation on the part with immersion gold boards then we get insufficient solder joints. We have many parts that are very old and oxidized given the nature of our low volume/high mix business.

Any thoughts?

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#33208

Gold versus HASL finish | 15 March, 2005

Comments are: * J-STD-001 and A-610 committees have eliminated the requirement that solder joints be "bright and shiny". Certain metallizations (such as gold) can influence the solder joint surface texture, but have no impact of reliability. Reliance on a solder joint's appearance as a measure of quality can be a dangerous assumption. * Minimize dullness by using nitrogen in your oven. * Minimize dullness by changing solder pastes. Some pastes provide for shinier solder connections than others. * Gold in the solder alloy increases the required peak temperature above that required for HASL [using the same solder paste] by maybe 5*C. * That the connector leads are not soldering well indicates that the leads are not getting enough heat or the flux is not active enough.

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#33210

Gold versus HASL finish | 15 March, 2005

-If the aperture volume is the same, there will be more solder on the HASL finish joint due to the solder already present on the pad. This difference is not usually enough to cause insufficiant solder volume but will "look different". Sometimes an inspector that is used to seeing a HASL finish joint will fail the gold finish because it is different. -If you have oxidation on your components maybe you need to change the way in which you store components.

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#33214

Gold versus HASL finish | 15 March, 2005

Mat, Are you sure you should be using SnPb HASL boards if you are doing prep for lead free? If you aren't actually going to comply with RoHS then fire away. Nearly all of our assemblies are now ENIG and as any re-design comes up on a HASL board it is changed to ENIG. As you say, the wetting does not always seem as good and the joints may not be as shiny but davef and jdengler have pretty well dealt with that. Oxidized components may require a more active flux than what you are using. Why change to ENIG? It's cheaper. What to trial instead of SnPB HASL? I'll leave that to our associates.

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KEN

#33220

Gold versus HASL finish | 15 March, 2005

The hasl joint looks fuller (and is easier to inspect)because it has more solder volume.

Think about it. Your pre-plating your board with solder (before you add more solder). I doubt your getting 0.3 mils of hasl.....probably more like 1.3 to 3.0 mils of hasl. This increases the volume at the lead (which is typically the hottest part of the joint). Hence the joint looks fuller. The gold may produce a frosty appearance to the joint depending on your gold thickness (5-30 micro inch?). Gold is instantly disolved into the solder and may change the appearance features of the joint. Excessive gold scavenging will cause joint embrittlement. But this is typically in joints that see over 5% (wt) gold.

The thermal profile is always important. I can not stress this enough. however, as a rule of thumb hotter (relatively speaking)tends to be better when soldering to nickel. Avoid the low side of your process temp. window. Also, aggressive fluxes will assist the cleaning and promote wetting on Ni. However, if you have good wetting spread on the gold you're probably ok (temp wise). If you have a single homogenous solder mass, you're probably ok. But, again, the profile can not be rulled out.

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