Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

intro to 2 sided assembly


intro to 2 sided assembly | 30 November, 2001

After saying for years that we'd never do two-sided assembly, the powers that be are not only saying we will, but they're threatening to throw an out of house design at frame (it's a PCI card with a couple of dinky breakaway tabs), no tooling holes, a nice combo of 240, 208, and 100 pin fine QFP's on the top, and a smattering of 100 pin QFP's AND 36 pin SOJ's on the bottom along with little stuff.

I said no way, they said "It'll be fun!"

One question, before I pursue this any further. Can I expect my SOJ's and QFP100's to behave during the second reflow, or will I need to dispense adhesive to keep them from moving around or littering the floor of our Heller?

I've heard conflicting answers to this question so I'd like to hear from someone that's either tried it and failed, or is currently having great luck with it.

These will be low volume builds so a nice purty inline dispenser is doubtful, for what that's worth. I'm thinking more along the lines of a somewhat controllable manual dispenser (such a thing??) if I even need one, but who knows...they could also decide they want to do everything new two-sided and want to splurge. Thanks.

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intro to 2 sided assembly | 1 December, 2001

Sounds like a perfect place to start. Sure we all have different perceptions of fun, don't we? Trial by fire, eh? ;-)

A prediction: Your cost accountants are going to freak when they compare the production costs for this board to the more familiar the single sided boards.

Peter Pan Components. There is a relationship [that we�ve talked about before here on SMTnet] between the weight of a component and the surface area of the solderable pads that determine which second side components go bug-up on the floor and which remain on the board. Bob Willis and Phil Zarro have published their thoughts on this on their sites.

Dispenser Suppliers

�Manual� dispenser suppliers are: * EFD 401.434.1680 * I&J Fisnar 2-07 Banta Pl Fair Lawn NJ 07410 201.796.1477

We use a �calking gun� manual dispenser for jobs like this. You know, the same as you use for manual soldering. Just hit the edge of the soldered component with a little glue and then run your paste on the primary side.

Programmable offline dispensers suppliers are: * TechnoISEL, 2101 Jericho Turnpike, NHyde Park, NY 11040 516.328.3970 fax 8827 * Online 815.363.8008 (RC1818) * DCI, One Kenwood Circle, Franklin MA 02038-9908 800.341.4811 fax 508.520.1424 * IJ Fisnar 201.796.1477 fax 794.7034

Changing gears, consider: * Adding rails with tooling holes to the boards. * Running these boards on pallets through your process from print through reflow and avoiding the discussion of adhesives entirely.

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Jason Dill


intro to 2 sided assembly | 4 December, 2001

I have successfully reflowed 208,100 pin QFP'S and SOJ 28's on the bottom side of the board. The only thing i am "SCARED" to reflow on the bottom side are BGA'S.

We used to turn off the bottom heaters but we did a trial run with them off and the board has ran perfect for 3 years now. Just try it. Like the other guy said Trial by Fire :)

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intro to 2 sided assembly | 4 December, 2001

Hi Steve,

It sounds like your product will have many moisture-sensitive devices on both sides. You should be aware of the additional controls that are required to insure the reliability of these components in a double-side reflow application.

The issue that is often overlooked is that you existing MSD tracking system must be expanded to keep track of the remaining floor life of the components between the first and second reflow. This time must be added to the exposure time prior to placement. Contrary to popular belief, the first reflow does not remove moisture from the MSDs. As a result the clock of exposure time is not reset and it continues to tick.

Ref. J-STD-033, section 8.4.4. "If more than one reflow pass is used, care must be taken to ensure that no moisture senstive components, mounted or unmounted, have exceeded their floor life prior to the final pass."

Francois Monette Cogiscan Inc. 50 De Gaspe, Suite A5 Bromont, Quebec, Canada, J2L 2N8 Tel : (450)534-2644 Fax: (450)534-0092

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