Wednesday, September 5, 2012

The joke's on me

When I was just starting layout I asked my friend Peter whether he thought I could consider using 0805 resistors, or if I should play it safe and use the larger 1206 size. During a rambling, friendly conversation he opined that 0805s would be fine, but he found hand-soldering 0603s wasn't really that difficult. He did advise me to stay away from the 0402s, not that I'd ever considered them.

As I worked on the initial DRAM array layout I hadn't yet decided to stick the vias in the corner of the pads. I was having trouble finding places for the vias and the 0805 capacitors seemed to take up a lot of room. Taking Peter's comments to heart I tried the layout with 0603s, and it made things fit much more nicely. Just to reassure myself that I wasn't being stupid, I dug out a dead board a client had given me, and convinced myself that this would work.

My long-delayed Digi-Key order finally arrived this afternoon. Along with the solder paste, it contained the passive components for this project. Eager to see what I'd gotten myself into, I took a roll of resistors, peeled back the plastic cover tape, and tried to remove the device with my fine tweezers. And found the pocket was too narrow. I turned the tape over to drop it onto a piece of paper and promptly lost sight of the thing. Eventually I noticed a rectangular dark spot and carefully centered it under my microscope.

Here's a photo I managed to shoot of the bugger through the microscope. This is an image I really suggest clicking on to enlarge. Those faint blue lines are the printed divisions on a sheet of graph paper. They're 0.1 inch apart. And you could put three of these things side-by-side in that square.

I guess I knew intellectually that 0603 meant that it's 0.06" by 0.03 inches. I'd even looked at some on a PCB, but handling them loose is a new experience. I feel like I need to wear a face mask lest I sneeze and blow it away (or worse, inhale it!).

When I was looking for a cheaper alternative to the BSS83, I considered the Diodes Inc. DMN26D0UT. But at 1.6mm x 0.8mm, I thought it was too small for easy handling. This thing is even smaller (1.52mm x 0.76mm), if only by a hair. The BSS83 and FDV301 transistors I settled on are twice this size.

Did I mention that Peter is an electrical engineer? He's one of the designers I worked with at a previous job. When he says these things aren't hard to work with, he's talking about something he does for a living, not as a hobby.

Still, all is not lost. The DMN26 has three tiny leads protruding from the sides. Each end of this thing is a lead, making them much easier to position and solder. There are only 87 resistors on this board, compared to 418 transistors. By putting a tiny dab of solder paste on each pad and then putting the device onto those, I should be able to hold the thing with one hand while I wield the soldering iron with the other. And they only cost a fraction of a cent each, so if I trash a few learning how to manage them, it's not a concern.

Most of all, it's a learning experience...

No comments:

Post a Comment