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January 1998

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Subject:
From:
Eddie Brunker <[log in to unmask]>
Reply To:
TechNet Mail Forum.
Date:
Mon, 12 Jan 1998 17:21:28 GMT
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text/plain
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>
>3.  We have chosen immersion gold as the finish for all production volume
>pcb's with 20 mil pitch or boards with lots of 25 mil pitch devices.
>

>
>5.  We reduce 20 mil stencil apertures 10% in length & width.  We use a 6
>mil mono-thickness stencil.
>


Thanks for sharing the detail of what you do.
I agree it's interesting know what others are doing.
I don't agree with the reduction of fine pitch in the length. My experience
is that there should be a reduction in the width but the length should be as
long as possible. The width reduces the proximity of the solder strips thus
reducing bridging. The length is then critical to providing sufficient
solder to form a good joint. The solder effectively wicks back along the pad
to the termination. The longer the pad, the more solder paste you can put
down along it and then the more solder in the joint around the termination.
Once you keep it in thin strips, then shorts don't occur.
As you continue to extend the pad beyond the toe of the TQFP then better
joints can be formed and more repeatable.
The rules for pad geometry in my book are:-
Min outer dimension with the max contact length should still allow for X2
the max lead thickness behind the heel. The max outer dimension should be
extended by X1.5 the max lead thickness, with any extra after that being a
bonus.
The width of 20 thou device pads should be as someone already stated, 12
thou with apertures of 10 thou. 25 thou depends on the lead width but we use
16 and 14 thou pads. The apertures being 14 and 12 thou. I make the pad
width the same as the max width of the lead, which leaves no side fillet at
that dimension, but usually they follow the nominal width.
The length of the pads using the above usually amounts to more than 70 thou,
but if it doesn't then I reccommend extending the outer dimension to make
them so.

As I have said before, I don't believe it is neccessary to have an
alternative finish to HASL for fine pitch. I use HASL for boards which are
heavily populated with 20 thou including connectors at this pitch, with very
high yield. The key is in the upfront engineering of the pad geometries and
apertures.

If there are different suppliers of devices, make sure all parts are vetted
by you with Data sheets so you can calculate the above dimensions. Obviously
the inside and outer dimensions will need to encompass the range created by
multiple sources. But don't let purchasing bring in parts that you haven't
approved.

Thanks again for sharing the details, I wish more Technetters would do the
same, then it would be easier to answer their questions, and we would all learn.

Regards

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