A PCB for your project -- part 3

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In this 3rd article on the topic of creating your own PCB, we’ll look at assembly.  One of my first columns was about assembling PCBs with surface mount components.  I’ve given it a try now, and I’m happy to report that it actually works.  In the photo you’ll see what appears to the untutored eye to be a hotpad.  Regular Gadget Freak readers however will recognize this as a cleverly disguised SMD conduction reflow platform.  These things turn up in the strangest places — I found mine in the housewares aisle at Target.  This reflow platform is the key to assembling your own PCB with surface mount parts.

For review, please turn to the SparkFun page in your browser and read “Reflow Skillet“.   In this article they used a skillet, which has uneven heat due to the thinness of the skillet material and the fact that the heating element is a single loop.  Their Target must not stock the Aroma SMD reflow platform because it’s clearly the better choice.  It provides even heat and gets up to temperature quickly.  Professional ovens have a specific temperature profile, heating everything to a low temperature first to drive out any moisture, then ramping to soldering temperature, maintaining it, and ramping back down at specific rates.

I tried to emulate that profile on my hotpad SMD reflow platform, but even on a relatively low heat setting it got right to business, melting the solder within a minute of turning it on.  Of the three surface mount IC’s, only one wasn’t perfect.  I hit those leads with a soldering iron and some desoldering wick and they cleaned right up.

But I am getting ahead of myself.  Once you have the blank PCB back from the manufacturer you have to prepare it for the SMD reflow platform.  After you send the gerber files to the PCB manufacturer you don’t just sit and watch the clock, waiting for the FedEx guy to show up with your boards.  You go over to pololu.com and upload the stencil file from Eagle and have a soldering stencil made up.  Pololu will cut the stencil for you from either 3 mil or 4 mil plastic.

Once you have the stencil, and the board, and all your parts, and some solder paste (I used a small tube of the stuff from  AIM Solder), you place the stencil over the part, apply some paste to the stencil, and squeegee it around.  I used an old plastic ID card that happened to be in my desk drawer.  You can do better than that.  Next time I’ll get a flexible stainless steel putty knife from the hardware store. You’ll also want to “pad” your PCB around the edges with other blank PCBs or perhaps cardboard if you can find the right thickness.  If the stencil drapes down over the edge of the board to the desk it will flex as you squeege, allowing paste to get between the board and the stencil.  This is what happened to the IC legs that I had to clean up. Once you’ve filled all the holes with paste and have scraped off the excess, remove the stencil, place the parts, and bake until well done.

Once again, SparkFun to the rescue with a tutorial on stenciling.  Read it, as they have filled in all the details that I skipped over.

Good luck with your PCB

Steve Ravet

Design News Gadgeteer

Send in your gadget! Design News is always looking for new gadets to feature in the print edition of the magazine.  Mechanical, electrical, pneumatic, whatever.  Believe me, if they’ll run my hot rodded Billy Bass they’ll run anything.  If you’ve creating something clever and would like to see it in print, submit it to the Design News editors:

http://www.designnews.com/file/25333-Submit_Your_Gadget_Freak_Design.pdf

You’ll get the thrill of seeing your product show up in a print magazine right in your mailbox, and you’ll get a check for $500 as well.