| | | | | Is there any problem using SMT on gold boards? 15 years ago we were told not to use gold boards, but I can't remember why. Judging by the other postings it now appears to be common. Only problem I see is that the nickel should be plated, not electroless. My vendor wants to switch to gold to get around some problems with the key contacts that currently use carbon. | | | | | Hope this isn't too elementary. I am a design guy, not a process guy. | | | | | Thanks, | | | | | Paul | | | | Paul- | | | | In our process we use both carbon and Gold fingers for our switches, The biggest saving you'll see between the two is, no more carbon voids or bridging. | | | | The gold will also carry a longer cycle life than the carbon. As far as the component pads are concerned, we run one board that is all gold plated, and the only manufacturing issue that we've run into is the added cost...copper or nickel is cheaper. | | | | -Ben | | | | | | Folks, | | | I want to reiterate my chief concern about electroless gold over electroless nickel. I have advocated the process since its inception. I posted this objective view in the June (now archived very kindly) forum. The following reply, and personal email, was received at that time: | | | There has been a problem with nickel/gold plating both on PCB's as well as BGA package substrates. The problem only occurs with electroless nickel/gold plating. In electroless nickel plating, phosphorus is added to the plating baths. When you solder to the nickel/gold pad, the phosphorus migrates to the surface of the nickel, forming a very brittle phosphorus/tin intermetallic between the nickel and your tin/lead solder. If you use electrolytic plating you should not have a problem, since phosphorus is not necessary to plate for nickel with this plating method. Many of the early BGA's used electroless nickel/gold plating on the substrates solder pads. Those suppliers who are aware of the problem have switched to electrolytic plating. You should check with your BGA component supplier as well as your PCB supplier. If you have any questions you can call me at 512-933-5783. | | | This is more than cause for concern. Therefore, I urge us all to look more closely and deeply into this subject and the processes required to effect acceptable solder joints. | | | Earl Moon | | Earl: If the villan is phosphorous in electroless plating, then do we have to be concerned about gold thickness? Do we want to maintain 2 to 7 microns or can we go back to more traditional thicknesses? | | Dave F | | Dave, | There are two villans if we go back to more gold as before. Gold still must meet requirements clearly indicated, as you know so well, in ANSI/J-STD-001B, Figure 5-1. If not, we're causing failure through the same old embrittlement problem. | The phosphorous issue is separate, but is cause for great concern. Industry is just beginning to address it, as I've been saying, but no resolution but for alternative immersion process additives such as Boron. However, this seems to have the same effect but with a different element providing the intermetallic formation. | I appreciate your addressing this issue. It was a very good question and I certainly wish to see us all get involved or the results may be as catastrophic as Y2K. | Earl Moon
Dave, I should clarify the above by stating gold plating or coating thickness should not exceed approximately 7 millionths or the tin/lead solder will be contaminated to the extent is exceeds the ANSI/J-STD-001B, Figure 5-1, requirements. Earl Moon
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