Testing the castellated module format

In the previous post working on the modularisation of the MEGAphone with NLnet Foundation's support, I described the castellated  module format I have developed.  The sample PCBs from PCBWay have turned up now, so time to try out how well it works in practice!

Here is what the sample boards look like: 

 

 

For ease, I made it a single panel, and made the little test modules snap-offs.  The test modules just have simple wiring from each pad to a set of hole-throughs in the middle, for testing continuity.

There is unfortunately a fabrication error on the PCBs: The little modules have all their cut layer a bit to one side, or perhaps the other layers in there were shifted. I have checked the Gerbers, and it was all correctly aligned there. So I'm guessing PCBWay might have accidentally moved something when setting it up, perhaps with the castellation, thinking that the far end of the PCBs should be castellated, instead of just the sides.

This has a few consequences: 

 

1. Pins 5 and 10 on the little PCB at least have their trace to the hole-throughs cut, so continuity cant best tested over those.

2. The cut-outs for the DIP IC puller to allow the module to be held still while soldering don't work, because the top and bottom edge of the module is misplaced by about half a pin.  This wasn't a huge disaster, but did make soldering it on fiddlier than it should have been. 

3. Also for desoldering, this means you don't have a nice way to hold the module with the IC puller, while using either a soldering iron or hot air gun to reflow it.

 

You can see what has happened with the misplaced cuts when the module sits on the carrier board: At one end there is spare PCB, and at the other end, there is only a 1/4 of a hole, instead of a half of a hole!

 

 

I did try filing off the excess at the other end to make it possible to use the DIP IC puller, but it didn't really help, as the opposite end was loose, and so it could rotate a bit. But every indication is with that fixed, it should work nicely.

 

The only other error I have found so far is a problem of my creation: The wide 20-pin module is too wide for the carrier board footprint, as you can kind of see in this image:

 

 

Whereas the other two smaller modules sit nicely over their footprints, the wide one is too wide, and doesn't leave enough of the carrier board pads visible. That will be an easy fix, however.

 

Time to try soldering it on, including fitting the VCC and GND straps using bits of solder wick.  That worked really easily, as the solder wick really wants to connect to the solder, even if it only reflows a little, because of the thermal mass of the carrier board's GND and VCC planes. So that feature is a win!

 

 

I ended up with it a bit rotated first time around as you can see above, and so I got to try out the ease of desoldering straight away ;)  With the VCC and GND straps not fitted, there was no massive thermal capacity problem, and with a bit of mucking about with my hot air gun, I was able to pull it off again prying it from underneath with a pair of tweezers. I was not at all gentle, and had the hot air set to 350C, which is pretty hot, and it still worked to remove it, without ripping any pads. You can see that some of the pads the solder was only softened, not properly melted when I pried it off, but all the pads still stayed put!

 

 

In the process of doing all this, I have learnt that the hole diameter in the castellated pads should be a bit bigger, so that the soldering iron can sit in it easier, to ensure ease of forming the solder joint. Also, for ease of removal, I should reduce the size of the pad on the underside of the modules.  The big pad on the top side is good, though, as it allows for continuity testing of the joints super easily using multimeter probes, because the pads are big enough to park the probes on each.

Remounting the module wasn't too hard, although I did learn the hard way, that its much easier if you first wick off the solder lumps from the carrier board pads and the underside of the module, so that it could sit flat on the carrier board again. This is another reason to remove the excess pad size on the underside of the modules.

 

Once I did that, it was really easy to reseat the module.  Holding the soldering iron vertical in the half-hole of the castellation seemed the easiest way to get both parts of the joint hot quickly.  

Again, I purposely didn't take too much care with this process, to simulate someone doing this with limited skills, or in an austere environment.

 

Then it was time to refit the VCC and GND straps. I recycled the saturated bits of solder wick from the process of cleaning the module and carrier pads, so it looks a bit scrappy, but the key point is that all connections have continuity.

 

The only thing to be careful of when recycling saturated wick like this, is to make sure it sits low enough, that it can't snag on anything sitting above the module PCB level, for example, if we had the carrier board sitting directly behind the battery or screen in the MEGAphone.  But its fairly easy to poke it down using a soldering iron.

 

The final thing to try out was to apply glitter polish, and see how the QR-code style reference points work to make it easier to acquire and compare an image of the glitter over the module. Actually making the image registration software to do this is outside of my scope, but if anyone would like to give it a go, and see how well it works, here are a few images from different angles:

Anyway, the short story is that this module format seems to be workable, and I just need to do a respin at some point with the reduced pad size on the underside of the modules, and fix the width of the wide module. I'll do those two when I design up the first modules (which will probably be DC:DC regulator modules), which will also give some time for me to figure out with PCBWay how the production fault occurred, so that we can avoid it in future.