Jonathan,

Forgive me if I misunderstand.  As I understand your statement, the top
metal remains solid and never becomes liquidous.  If so.... an air bubble is
less dense than the molten solder, so it will rise to the top of the liquid.
When it reaches the top, it will dissipate, unless there is something there
to trap it.  The top metal will trap the air bubbles.

Best Regards

Ryan Grant
Advanced Technology Engineer
MCMS
(208) 898-1145
[log in to unmask]

> -----Original Message-----
> From: Jonathan A Noquil [SMTP:[log in to unmask]]
> Sent: Friday, May 11, 2001 3:56 AM
> To:   [log in to unmask]
> Subject:      [TN] Solder Voiding
>
> Hello Everyone,
>
> Please help,
> Can Anybody explain why too much voiding
> occurs when two metal is joined (Top Metal
> and Bottom metal)? Eventhough the peak temp is
> 40 C over than the liquidus temp of the solder.
> When a solder is reflowed without top metal
> the solder wetting is very good and the appearance
> is shiny. But when you put a top metal to be joine
> and then X-ray, there are a lot of voids.
> I tried to removed the top metal, and the picture
> below showed a lot of voids. Another thing is the
> color is not shiny but grayish.
>
>
>
>
> (Embedded image moved to file: pic29915.pcx)
>
> The paste is 85Pb/10Sb/5Sn
> Peak temp is 290 C
> Oven: Conduction
> Time until the unit is out: 180 Seconds.
>
>
> Many thanks to everybody.
>
> Sincerely
> Jonathan
>
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