Screen Print and Reflow SMT Boards at Home

6. Screening a Double-Sided Board, Side Two.
After populating and reflowing the side with the smallest components, repeat the process on the side with the larger SMT parts. This sequence will keep the largest and heaviest parts from hanging upside down during reflow, reducing the likelihood that they would fall off. Molten solder has very high surface energy and can hold most SMT parts even up side down.

Print the second side with the PCB on cardboard* that is large enough to attach the L-shaped board holder and your board assembly. Use cardboard that is thicker than the populated PCB and thick enough to function as a support for the screening process. *Cardboard is the easiest, but you can use wood or any non-static generator.

The cardboard should be cut out to clear the SMT components from the first reflow. Again, make sure all work surfaces are ESD safe or grounded.

Tape the cardboard "holder" to the work table and do everything on top of the cardboard the same as you did when screening solder paste on the first side. The reflowed components nest safely in the openings in the cardboard. The board rests flat on the cardboard with the surface to be screened is open to the prototype stencil.

7. Reflowing side 2
When you reflow the second side of the PCB, the whole assembly has to be held off the oven shelf with standoffs. You'll damage the SMT solder joints from the first reflow if you lay the board directly on the oven shelf.

You can use any material that can withstand the oven temperature to create stand offs to hold the board, but the very best is scrap FR4 material. You probably have some old board scraps laying around, but if not, just prop the assembly up on a couple of pieces of metal or anything that is not going to be affected by the heat of the oven. I used large binder clips for this board and they worked fine.

8. Touching Up Defective Solder Joints
Touch up any solder defects with a solder iron. Use a fine tip on the solder iron and don't let the tip touch the body of the SMT parts. Set your solder iron temperature as low as possible. A solder iron that is set to a high temperature can thermal shock SMT components.

That's it. You now have your finished boards, ready to test and use in your latest project. Make sure you handle your SMT and mixed technology SMD board(s) carefully. They're not as rugged as 100% through hole, but what you can create with them is far beyond anything you could ever get with only through hole. Experiment. Have fun.

Questions? Contact me and I'll do my best to help.
Bob Rook, SCT Engineering
513 Sweetleaf Dr., Brandon, FL 33511
813-505-0728; sctengfla@msn.com
for information on the Prototype Stencils:
www.pcbexpress.com/stencil
www.stencilsunlimited.com