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Home made PCB?
On Jun 30, 10:12 am, "numeric" wrote:
Hi, I would like to build a PCB board with fine pitch (LQFP32) and very small surface mount parts (1206 and smaller) parts. The board is a simple 24 bit A/D and D/A converter with a full speed (12 Mbs) USB2 interface; although I would prefer the USB2 high speed interface (480 Mbs). The temptation is too hard to resist using today's microprocessors such as SiLabs C8051F350 (basically an 8051 cpu). I know that a commercial 4 layer PCB with a solder mask would probably work for homebrew construction; but the board cost is high. So what is the chance of a double sided PCB without solder mask working with very small parts? The process I would use in either case would be: 1. Place solder paste on all pads. Typically, when heated the solder will vacate between pins and will suck up, like a magnet, under the SMD pin. 2. Place glue under parts that will possible move when the board is handled. 3. Heat the PCB in a toaster oven until soldered. Manually follow the heat up and cool down time curves for type of solder paste used. Comments/suggestions are appreciated. I agree with Leon and Tim that making a board and mounting parts on it shouldn't be a problem, though I do greatly appreciate being able to do my soldering under a stereo microscope using a Metcal iron. I commonly use boards coated with positive resist (from Circuit Specialists in the Phoenix area), etched by floating them on top of ferric chloride. I find that (lacking a spray etcher) the float process maintains fine line resolution--I have a theory that it has to do with copper-laden etchant being heavier and falling away from the etch site, but whatever, it works well for me. Surface tension will hold fairly large boards up, but you can also just put support at the corners and let the board submerge. Resolution for me is always limited by the quality of the transparency. Thanks to Leon for posting a reference to another film material to try. I've had the best luck using an Epson inkjet printer: my HP printer doesn't do as well, and laser printers typically don't print a dense enough black. Pinholes WILL print on the board! I use a back-light film setting on the Epson for highest density. An alternative is to get a photoplotting shop to do the film for you: better quality but it lacks the immediacy of being able go from layout to board in minutes. Anyway, the best inkjet printed transparencies will allow me to do 5 mil lines with 5 mil spaces if I'm careful, and 8/8 pretty easily. Most of my work is with 0805 as the standard passive, since that's what I invested in an assortment of back a few years ago, but I also do 0603 with no trouble, and lead pitches down to 0.5mm. We have fancy equipment to do BGA rework here at work, and the trained techs have marginal success with it at best (for large parts), so I've avoided BGAs in my home projects. By the way, if you get boards from Circuit Specialists, be aware that what they call 1 ounce copper is really 0.5 ounce...seldom a problem and allows faster etching and it's easier to make fine lines, but be aware of it for high current traces. Cheers, Tom |
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