----- Original Message -----
From: <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, March 15, 2004 7:05 AM
Subject: Re: [DC] Netlist?


> Wuz wondering.... Why exactly is it that we abandon HASL for ni/au or
> whatever on fine pitch designs? I know it's a finish flatness thing, but
> can anybody 'splain it to me? Doesn't the solder paste process kind of
> level the pads back out?
>

I can't explain it too well, but yes it is an issue of flatness.  Look
closely at a board with HASL vs. an immersion finish; there's a BIG
difference in the profile of the lands.
When the pitch becomes very fine, the possibility of bridging and slight
mis-registration becomes much higher with HASL.  Any application of solder
paste should simply add to rather than even out the problem.
Hopefully someone can site a more technical answer.
Richard

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