Screen Print and Reflow SMT Boards at Home

by Bob Rooks



A Surface Mount Overview

Every day seems to offer new Surface Mount Devices (SMDs) that tantalize you with a wealth of capabilities you can't find in through hole components. In fact, many types of through holes are getting harder to find as time goes by because Surface Mount Technology (SMT) is a more profitable process for large manufacturers. SMT gives them higher device capability and it's streamlined to fit right into automated assembly. Device manufacturers are following the money. Although it looks like some through holes will always be around, most are disappearing. There just isn't enough business in through hole parts to justify making most of them anymore.

Because the length of the traces are shorter on SMT devices, they offer a real advantage in high frequency applications. You also get more function on any given piece of real estate, which probably drives surface mount more than anything else.

Basically, what you can build with surface mount goes far beyond what you can build with through hole as the following SMT schematic illustrates.

Still, purely surface mount may never be a reality because even digital boards have connectors to plug them into the world. Connectors produce exceptional stress on the solder joints which is a negative for SMT. Compared to through hole, SMDs are easier to rip off the board. So, until somebody fixes this problem, components like connectors will continue to rely on basic through hole technology. In the plant, this means they wind up reflow soldering the SMDs and use wave or selective soldering for connectors and some analog devices.

The larger size of solder joints and wide spacing between leads on through hole boards are easy to handle for anyone familiar with hand soldering. Soldering the tighter pitch on surface mount devices is the main problem for a home workshop.

Enjoy SMD capabilites using
the at-home procedure outlined here.

In defense of SMDs, printed circuit board has glass fibers in it arranged in an X-Y direction, not in the Z direction. The coefficient of thermal expansion in X-Y is much less than it is in Z. So whenever you touch a solder iron to the board it expands more rapidly in the Z direction, causing stress in the barrels of plated through hole and via. This stress can crack the copper barrel where it joins the pad on the board surface. SMD boards are more reliable in this area than through hole boards.

Home Workshops and SMD Assembly Problems

You'd like to take advantage of some of the newer SMD capabilities, but may have avoided it because of the problems associated with hand soldering the leads. To compensate, many of you will have mixed technology boards built to your design spec and then send out to have them assembled at a contract shop. This can add a lot of time and expense to your project unless you're really lucky and have a buddy or a long standing relationship with a local shop that will put your one or two boards at the top of their production schedule.

You know that screen printing your solder paste and reflowing your assembly is the method you would choose if you could. But, to date, nobody has a screen printer or reflow oven that would even fit on your workbench - let alone, be affordable.

Well, guess what? Now you have both. The following method I'm going to take you through is far from the sophistication of major screen printing and reflow equipment, but it gets the job done - even double sided SMT boards. You can do it right in your garage or workshop. It's possible to actually cut your assembly time, increase the quality of your designs and open the door to all the mixed technology you want to experiment with.


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