Francesco, I looked into what sounds like a similar problem ~1.73-1.88 years ago. Technet contributors provided a lot of information and help, but we never reached a convincing conclusion. Best I can say for the moment is that I would hesitate to blame the solder paste too quickly. The next step in our work would have been FTIR analysis of the surfaces, which we did not perform. One thing I have learned in the interim is that a BGA pad has area less than or equal to the minimum that required to perform FTIR with standard instruments. I understand micro-FTIR instruments would do the job.

Maybe we will hear more from TechNet after tomorrow. I suspect many are at IPC EXPO.

Louis Hart

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Francesco Di Maio
Sent: Wednesday, March 26, 2014 11:02 AM
To: [log in to unmask]
Subject: [TN] Dewetting area after solderability test

Hello Technet,

I have performed a solderability test Surface Mount Simulation Test according to the J-STD-003C on samples of PCB ENIG finish.
The reflow has been done with vapor phase equipment using Lead free solder paste SAC305.
Solder paste was depositied with ministencil for BGAs (1mm pitch) areas and manually deposited on other different pads (CHIP 0603,0805, SOT,BTC components and others).
The paste in this case was placed with not homogeneous deposition on pad area I know that this manually deposition is not correct, but I didn't have the gerber file for stencil. 

The result was that on BGA areas there was a perfect wetting (100%) while for some pads of others components I got some areas showing dewetting.
On these pads, the surface of the dewetting area were around 6--8% of the pad surface (cumulative percentage divided in 2-4 areas) and in one case was around 15% (cumulative, 3-4 areas of the same pad) .

Really I think these results is mainly due to the uncontrolled paste deposition considering also that the lead free paste has a high contact angle and that it wet less all the surface finishing and not due to poor PCB finishing, since I found several pads well wetted. 

Looking to the standard I see that the requirement for the Pass Condition is a good wetting for at least 95% of the pad surface area, but there is also the statement saying "There shall be no nowetting area or exposed base metal within the evaulated area" .
This should mean that, if there is an area of nowetting (exposed base
metal) the test should be considered Fail also if there is a good wetting for more than 95% of the pad area?
In subject case I have a no-wet area of 1.65% I consider this as Pass condition.

Does anyone has experience on subject? Or suggestion ?

Thanks in advance.
Francesco
Francesco Di Maio
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