Tuesday, May 3, 2011

Backplanes in the mail...

Yay, when I got home last night the alpha series backplanes came int he mail (always a happy thing to get new boards).  This design pushes the manufacturing a bit so I was interested to see how things come out (planning and math never match a real object exactly).  These boards provide power (two larger supplies and 1 tiny battery supply) to a set of 5 connectors, they contain a processor to control the activation state of the attached cards (with status indicators optional) and link single ended and differential pairs between the attached cards. 

I put silkscreens on both sides (this one is technically the top layer but it's not the one the connectors and components go on).  This makes it easier to access for testing since the boards will occupy the other side and restrict access.

One of the not fun things about boards like this (with many connection points) is testing it to make certain the connections are all as intended.  It's worth spending a few hours doing it though since it is very hard to locate and diagnose problems after the boards are populated.

So after dinner I sat down and tested... All the ground connections are correct (connected to each other and not connected to the supply lines).  All the supply lines (1,2 and 3) are correctly connected to each point they ought to be and not cross connected to each other. (yay)

Some of the single ended lines on the cards are connected to the controller individually and those are correct.  Some of them are connected to each other as a bus as well as the controller and those are correct (no cross connections to gnd or power as well).

The signal lines that form the buses between cards are all connected correctly to each other and there are no cross connections to the gnd or supply lines or to their neighbors (this was tedious to test and I kept loosing my place so I started using masking tape to mark my positions after a while).

I also measured the resistance between connectors for the signal lines.  With the limited equipment at hand it always came out as 0.1 ohms for one connector distance (which is 1.5 inches), 0.2 ohms for two connectors, 0.3 ohms (fluttry with 0.4 ohms) for 3 connectors and 0.5 ohms for 4 connector distances (which is 6 inches).  Clearly I'm too close to the limit of the meter I was using, but it looks like about 1 ohm per foot (give or take).  If I chain signal lines to increase the distance it look more like 1.3 ohms per foot (linking 6 sets of 6 inches for 3 feet at 3.9 ohms).  This would translate to 0.16 ohms between the connectors (1.5 inches).  I guess this makes sense for 6 mil lines.

I also did some quick capacitance tests (low frequency, not high yet) and it seems that looking at the power connectors it's: 149pF between supply1 and gnd, 70pF between supply2 and gnd, 13pF between supply3 and gnd, 31pF between supply1 and supply2, 13pF between supply1 and supply3 and 9pF between supply2 ad supply3.  Since capacitance can change so much depending on the environmental conditions in this case, the absolute numbers are not as interesting as the relationships between them. (I subtracted the air distance only measurement so that these are just the effects due to the board itself.

I was kind of surprised at the uniformity of the capacitance of the signal lines.  The single ended lines that had connections to the controller all ranged from 17 to 23pF depending on the connector to gnd.  All of the individual lines connected only as a bus were 22pF to gnd.  All of the signal lines were 18pF to supply1, 15pF to supply2 and 8pF to supply3.  Column A always has singles ended lines and columns B and C form a differential pair.  For the rows with signal lines the cap. between A and B or A and C was always 6pF and between B and C it was always 10pF (making me suspect that the closeness of some of the lines around the connector pines plays a large role here).

Measuring the cap. from the supply and ground lines on the connectors gave about the same numbers as measures from the power connectors but varied up or down 1 or 2 pF depending on the individual pin and the connector position.  Again, these are more for my amusement and interest rather than stating hard results since I'm doing this in an uncontrolled environment with uncalibrated equipment - but the relative associations ought to be the same).


As with the previous board from Laen's service (DorkbotPDX), the copper layer seems far more accurate than the mask and the silkscreen has the lowest resolution and variability (this is as it should be).  The smallest lettering is easily sufficient for assembling the tiny parts.  I'll get it on a scope and look closely at the traces but electrically everything seems correct so I don't expect to find anything wrong at all.  I did notice that the alignment of the solder mask makes some of the holes not to appear as centered as they really are - I could increase the size of the unmasked areas, but then that would increase the chances of an unwanted solder connection.  It's more obvious (mask intrusion) where I did fills and cutouts of the planes - it all looks good though and it's better to have more area masked than not enough (especially with tiny traces running between the holes).

I'm quite happy with the quality and hopefully I can get a board populated so I can start injecting signals soon.