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May 2000

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Subject:
From:
Mike Lang <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 8 May 2000 10:26:15 -0400
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Hi gang,

A few general questions regarding fine pitch SMT assembly:

1) My research indicates the "industry starting point" for fine pitch (0.020" pitch) solder pad and stencil design is 0.012" wide PCB solder pads and 0.010" wide laser cut stencil apertures in 0.005" or 0.006" thick stencils.  I find that using these parameters I still tend to get some solder shorts.  Any comments about better pad / aperture designs?

2) I am using HASL, but recognize the benefits of going to OSP (flatter among other things).  My concern is the bare copper left on the PCB since the paste does not fully wet out the pad.  Does anyone have comments, or better yet, test data which demonstrates what is acceptable?

3) Another HASL / OSP concern...  PCB is double sided reflow using no clean, and then has a small quantity of hand soldered (GASP!) through hole components assembled.  Will the PCB remain solderable for hand soldering after 2 reflow cycles?  Again, I expect bare copper in places since the solder will not naturally flow out to cover all surfaces of the hand soldered pad, top and bottom, especially after two reflow cycles.


Thanks!

Lang

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