Trevor,
If you have access to sectioning equipment, try this. Cut out a section
that has a good assortment of glued or known bad parts on it. Cut and grind
off any topside parts until the L1 layer is flat. Pot the section, with L1
down - so it'll face the grinding wheel. Take the potted section, and grind
up, through L1, toward the bottomside of the board. It's easy to keep this
planar section flat because the different layers should uniformly appear
across the section, and then uniformly disappear as you keep grinding toward
the next layer. When you get to the last layer, and the soldermask beneath
the glue dots, go the rest of the way with very light pressure, and lots of
water with a worn 600 disk. Finish with a diamond mesh, until you are left
with only the soldermask and the glue dots between the component mounting
pads of the bottomside parts which are still embedded in the section. With
practice, you can remove all the mask, or get it so thin that it's virtually
transparent. If it stubbornly adheres to the section, you may want to flake
it off with a sharp pick.
You might find that your glue dots look something like the cross section of
a tomato. The hygroscopic characteristics of your adhesive (not all
adhesives are created equal in this regard), the surrounding environment -
especially the humidity, and the curing profile are all critical factors in
this problem.
Bruce Tostevin
Benchmark Electronics
Hudson, NH
> -----Original Message-----
> From: Trevor Goddard [SMTP:[log in to unmask]]
> Sent: Wednesday, November 01, 2000 12:08 PM
> To: [log in to unmask]
> Subject: [TN] solder under chip components
>
> Has anyone ever seen solder shorts under an smt resistor that has been
> glued on a board and then wave soldered?
> The boards go through the standard surface mount process of screen
> printing
> the glue, placement of components and curing the glue. Then the boards
> receive solder reflow for the top side of the board, on to hand assembly
> and
> then wave solder. During all in process inspection even after wave solder,
> the boards look good. When the boards get to test it is a different story.
> Randomly across the bottom side of the board, the side that was wave
> soldered, we will get short conditions on various caps and resistors. If
> you
> look at the component nothing appears to be wrong, but when the component
> is
> removed you can see solder on the under side of the component and on the
> pcb.
> This problem has just cropped up, so I find it a complete mystery.
>
> Thanks in advance.
>
>
> Trevor Goddard,
> SMT Supervisor
> XLTEK
> (905) 829-5300 x 348
> [log in to unmask] <mailto:[log in to unmask]>
>
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