One way to stop the solder wicking without plugging or tenting the via holes is to extend the solder mask, so it would cover the via pad without encroaching on the via hole itself. This way there will be a wider dam of solder mask that should be enough to prevent the solder thieving.
Sherif Refaat, Eng.
Technology Roadmap Consultants Inc.
E-mail: [log in to unmask]
Tel. (514) 624 9842
Fax (514) 624 0091
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard
> Sent: Wednesday, November 30, 2005 3:59 PM
> To: [log in to unmask]
> Subject: Re: [TN] Plugging vias necessary?
>
> Without the plug, the solder from the sphere can wet down into the
> adjacent via to the backside of the pwb during reflow.
> On a dogbone pattern, with one side the pad and the other side the
> connecting via, the small soldermask dam that goes over the thinnest
> part of the dogbone pattern is not enough to stop the solder wicking.
> The solder will wick right under that flimsy little dam, and the via
> will rob the solder. This is called solder thieving. The soldermask plug
> prevents that from happening. If you ever need to rework that BGA, the
> site prep operation nearly always makes the little soldermask dam worse,
> and after you replace the part you get even more solder thieving, and
> the entire rework/X-ray/test/rework cycle spins on down into a black
> hole called scrap. One almost never sees BGAs with daisychained dogbones
> without the soldermask plug, and that is a good thing, as Martha says.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Black, Paul
> Sent: Wednesday, November 30, 2005 7:50 AM
> To: [log in to unmask]
> Subject: [TN] Plugging vias necessary?
>
> Hi Everyone,
>
> I have a board with a 357 pin PBGA. Each pad has a short etch leading to
> an adjacent via. Currently, the fabrication drawing calls for each via
> to be plugged. The assembly process for this board does not contain a
> wave solder step and the test engineer assures me that plugging the vias
> is not necessary for vacuum during incircuit testing, so is there some
> other reason for plugging these vias? They are plugged with soldermask,
> and I feel like we are unnecessarily introducing potential problems into
> the process.
>
> Thanks in advance for your opinions.
>
> Paul Black
> Manufacturing Engineer
> Kronos
> [log in to unmask]
>
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