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January 1999

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Subject:
From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 15 Jan 1999 20:14:02 EST
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In a message dated 1/15/99 2:43:20 PM Pacific Standard Time,
[log in to unmask] writes:

<< We are assembling a small, panelized board that has all surface mount
 components on one side and all through-hole components on the other side,
 thus all solder is on one side of the board.

 There are 9 through-hole components on each board and there are 16 boards
 per panel.  Besides doing the SMT first and then coming back and hand
 soldering the through-hole parts, how are other people doing this?  Does
 anyone do the through-hole first and come back and do the SMT later?  If
 so, are you using a fixture to cover the pads for the SMT parts, or do you
 mask, etc?

 Suggestions and comments are greatly appreciated.

 Please email response to [log in to unmask]

 Thanks!>>

Hello there Bryan!

So you got yourself a type 2C assembly huh? Just for some info to store away
in the ol' memory banks:

Type 1A: single side mount, PTH only
Type 1B: single side mount, SMT only
Type 1C: single side mount, PTH & SMT
Type 2A: double side mount, PTH only
Type 2B: double side mount, SMT only
Type 2C: double side mount, PTH & SMT

"Okay, that's kewl Steve, but how do I build the darn thing?"

Okay, okay...don't get yer' panties all inna bunch, I'm fixing to tell
ya'...(GRIN)

There's a couple of ways you can go about this, and it depends on how the
board was laid-out, and what kind of equipment you have. If you're auto-
inserting your PTH, I tend to like to do that first, IF I have a glue
dispenser. That way you don't have to worry about the parts being knocked off
by the clinch and being subjected to all that banging going on down beneath
the board. You auto-insert, dispense the glue, place, cure, and wave.

If you don't have a dispenser, then you're left with only two choices, either
stencil the epoxy first, or print paste first, then auto-insert. You can do
either one, it's your choice. But for me, I'd much rather not deal with epoxy
if I can help it...solder is much more robust than epoxy, and you don't have
to worry so much about losing the parts you put on the backside in SMT before
the board gets to wave. Now for the big if...using solder paste and reflowing
the parts first, then auto-inserting means that you'll need a wave fixture to
mask the surface mount on the bottom and only expose the PTH leads. There's a
LOT of that going on nowdays, most of the shops that make wave solder fixtures
have gotten real good at making nice fixtures to do that.

But this can only be done IF there's enough clearance around the PTH leads to
be able to mill an opening for the leads and still be able to seal off the
rest of the board from the wave. For instance, if your DIP IC's have a
decoupling cap on the bottom in between the two rows of leads, that's gonna be
pretty darn difficult to machine around. But in those kinds of cases, where
I've had a board that has a few parts really close to PTH and make it hard to
mask off, I'll still use solder paste and make me a SMT masking fixture, but
I'll dispense a dot of epoxy between the pads at those locations that I wasn't
able to mask with the fixture, after the board comes out of the printer and
just before placement. I do that with one of them semi-auto, foot pedal
dispensers.

When you run the board for reflow, the epoxy will cure at the same time, and
you won't lose the part in the wave. If there's more than just a handful of
parts that are like that, I'll just say screw it and use epoxy for the whole
board...even though I don't like using it that much.

I hope this'll help ya' a bit! Have a good weekend!!

-Steve Gregory-

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