Gold pads have more then one purpose. If the gold pad is a contact connector 
subjected to repeated use, the electroless nickel / immersion gold process 
only deposits 2-8 micro inches of gold which may not hold up to repeated use. 
We have converted several customers from HASL to both E Ni/I Au and Entek. 
Cost and the end use of the board should dictate the surface finish required.




[log in to unmask] Wrote:
| 
| Scott
| 
| I cannot answer your question about the Enteck and the 
| tarnishing, but I am 
| wondering why you have selective gold plating and the 
| Enteck process.  
| 
| The electroless gold plating can be used to ensure 
| solderability of SMT 
| pads.  We presently populate and reflow a PCB that is 
| electroless gold over 
| electroless nickle.  The gold plate ensures the 
| solderability of the nickle 
| and the thickness of the gold plate is such that you do 
| not have a gold 
| embrittled joint..  We have been running this in 
| production for well over a 
| year with no solderability problems.
| 
| This process was chosen because we needed a nickle pad on 
| the PCB that 
| contacted a nickle terminal within the housing(like 
| metals) for wear 
| purposes.  Originally the PCB was going to be the nickle 
| plate with HASL 
| and the one pad would be masked off so it would stay 
| nickle.  The bare 
| board fabrication cost was less by using the nickle plate 
| and then the gold 
| plate.  The other process was lengthier and required 
| masking of the one pad 
| during the HASL step and then removal of the mask.  Since 
| gold is a noble 
| metal, we can leave it on the contact pad with no problems 
| and the gold 
| ensures the solderability of the nickle.
| 
| To re-ask my question--Why have a PCB with some pads gold 
| plated and the 
| other Enteck, thus requiring more steps in the PCB 
| fabrication when you 
| could have the entire PCB electroless gold plated and 
| still have proper 
| solderability.
| -------------
| Original Text
| From: 
| C=US/A=INTERNET/DDA=ID/TechNet-request(a)ipc.ipc.org, on 
| 9/18/96 7:30 
| AM:
| From: Scott Westheimer
| Date: 9/17/96
| 
| Sorry if my message of last night was not very clear, but 
| what can you
| expect at 2:30 am. 
| 
| The issue is with entek coating not the gold plating. The 
| panels have
| selective gold plating on some pads and the others have 
| entek. The problem
| is that when the panels are first gold plated( 
| electrolydicly ), then S/M 
| the entek pads are fine. However, when the panels are 
| first S/M and then
| gold plated the entek pads are tarnished around the edges. 
| Does anyone have
| experience with this problem.
| 
| Thanks,
| 
| PS Before anyone makes the comment, the boards are enticed 
| after all the
| processes have been completed.
| 
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