Wednesday, September 19, 2012

More Population, More Bugs

I've been debating how best to approach populating this board. It's certainly easier to populate the board if I can use the solder stencils to place solder paste, rather than have to put dollops of paste on each pad. Thus the limited flexibility of the stainless steel stencils dictates what areas I can easily populate. As tempting as it might be to just do the whole board at once, that's just not practical. I'm sure I'd manage to brush against the board and smear a few hundred components off their pads before I'd be able to get everything properly placed.

In the end I decided to get all the decode logic populated and tested, then work my way across the board toward the upper-left corner. So that's what I spent the evening doing.

Here's a crappy photo of a section of board partially populated, taken through the microscope. The upper row of components are 4.7K Ohm resistors in place. The second row has two FDV301 FETs and an unpopulated space for a third, while the third row is still unpopulated. You can clearly see the little rectangles of solder paste on the pads left by the stencil.

Remember, the larger components in this photo are half the size of a single grain of long-grain rice. This makes component placement a tedious, exacting activity. It's obvious why they have machines to do this. It'd be even worse if I had to then touch a soldering iron to each of these pads, but the Hot Air station makes it so much easier. I really am sold on this thing.

Here's the result, after everything was soldered down, and the board placed in the test jig. I'm not quite as good as a commercial pick-and-place machine followed by a computer-controlled reflow soldering system, but this is a hobby.

I went through the various outputs with my 'scope. I'm still not used to seeing RC charging curves, having worked mostly with TTL and CMOS logic that gives actively-driven, fast-rising edges. But if I mentally cut off the signal at the 2V mark, which is higher than any signal needs to go unless it's driving a transmission gate, things look pretty good.

Or it did, until I got to the Row Read decoder. That signal simply refused to go high. There are four inputs to the final NOR circuit; all need to be low for the output to go high. In this test configuration, two inputs are a constant low, and showed grounded on the 'scope. Two other inputs are pulses, and the output is supposed to go high when both of those pulses are low at the same time. I could see that happening at the appropriate time, but the output never went high. I tried reheating the connections of the four parallel transistors that make up the NOR circuit, but that didn't help. Eventually I removed the bottom two FETs -- the two whose Gate leads were being held low -- and the circuit started working. If you look really closely at the picture above you'll see where these two used to be.

I can see several possible causes. I could have had a solder bridge under one of the components, even though I didn't see a short with my ohmmeter. I could have damaged one of the FETs, either through static or by over-zealous use of the hot air. Or one of the FETs could have been defective. I'll check the board to make sure there isn't a short from drain to gate pads (which shouldn't be, if the electrical test was thorough) and then replace them with new parts.

I think there's a lesson here about laying out the circuits such that testable groups are separated enough that I can use the stencil without kinking it.

Update: The problem with Row Read decoder was that the POC input, which drives one of the four FETs, wasn't connected on the test jig and was floating. Putting a scope probe on the gate lead was enough to siphon off any charge and show 0V, but removing the probe allowed it to float high enough to turn the FET on and pull the drain to ground. Fixing this on the test jig restored full functionality.

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