My recent reading has indicated that the trend now is to use
non-solder-mask defined lands for BGAs. There is some evidence
that long term reliability suffers if solder-mask-defined lands
are used, as there is a stress concentration point where the
solder ball meets the edge of the solder mask.

In the interests of stress balancing, I match the land size to
that of the contact point between the solder ball and the BGA.

Failing the availability of that information, the next best
guess is to us a land that's 66% of the ball diameter.

Soldermask should be pulled back to the point that that the
collapsed ball won't be touching the SM (in an ideal world...).
This target is another influence on whether or not to use
dry film resist (its increased thickness compared to liquid
photoimagable mans it has to hav a larger SM to pad clearance).

John Haddy

> -----Original Message-----
> From: Tim Hutcheson [mailto:[EMAIL PROTECTED]]
> Sent: Friday, 24 August 2001 6:24 AM
> To: Protel EDA Forum
> Subject: Re: [PEDA] tenting vias
>
>
> What about the issue of whether to use a solder mask around the BGA ball
> land patterns?  I though I read somewhere that there are pros and
> cons about
> this as it might cause ball breakdown during placement.  If so, what would
> be the correct SM diameter to use with a 25-mil ball land?
>
> Tim Hutcheson
> Institute for Human and Machine Cognition
> 40 S. Alcaniz St.
> Pensacola, FL.  32503
>

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