I have to agree with Liang. If the printed solder paste melted at all, some of the solder should have wetted to the board pads.
However, solder follows the heat. If the pad temperature is too low, but the component is quite hot, theoretically the molten solder could wick up onto the component terminations. However, I really doubt that is the case, because apparently there is enough heat to get the solder paste to melt, and that should be hot enough for the solder to wet to the pads as well. Total nonwetting of the solder to the pads indicates a solderability issue with those pads.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Liang Yin
Sent: Friday, November 09, 2007 10:59 AM
To: [log in to unmask]
Subject: Re: [TN] non wetting
Lon,
What your PCB producer told you does not make sense.
Although the tin-lead alloy might have different wetting performance on flash gold pads, it should not have non-wetting in any event. If the flash gold is still there after soldering process, you have solderability problem with the boards. X-ray inspection is not a solderability test.
Liang Yin
Process Research Engineer
Unovis Solutions
> Date: Fri, 9 Nov 2007 16:23:00 +0100
> From: [log in to unmask]
> Subject: [TN] non wetting
> To: [log in to unmask]
>
> Dear Technetters,
>
> We had two boards out of 25 with a nonwetting problem. After reflow there was no solder on the flashgolden pads. Instead all the solder had gone to the leads of the 0805 capacitor.
>
> Our pcb producer comes with the following conclusion:
>
> "Our Q-dept. just informed me that they did x-ray inspection of the 2 boards you sent us back recently. Apparently there have been components used which contain up to 20% lead. During reflow-soldering the tin-lead on those components will melt first and absorb all the solder paste even before the tin which is included in the solder paste will melt. A golden flashgold pad without any tin will be the result. The tin will accumulate on the components and create those solder-balls instead.
>
> In a second test we did lead-free soldering on the side which contains no components. The two PCBs showed excellent tin-wetting on this side. All pads - big and small ones - are nicely covered with tin. So basically we will reject any complaint - the boards are ok."
>
>
> Does this make sense?
>
> Regards,
>
> Lon Weffers
>
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