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Fixed board revision sent to fabrication

A project log for uCM4

Smallest practical Compute Module 4 carrier board for network projects

Kamil LorencKamil Lorenc 07/13/2021 at 16:030 Comments

Increasing interest in my project motivated me to finally make next revision of PCB. Let me show you version 0.1.1. This is solely bugfix version and gets rid of most of the mistakes that I made on first iteration. There is nothing new, though. Goal here is to end up with a board that I can safely solder myself and use for further testing, possibly making 2 additional boards and trying some experiments in setting up Kubernetes.

Anyway, I fixed power rail width error, increased diameter of Ethernet shield hole, so now it should be way easier to place and this time I didn't forgot to regenerate drill files :) so now testpoints for USB should be on its place, too. Log of important changes is now on Github release page and obviously full log of changes is in Git's commit history. Here is the visualization of these few changes. First are drills (green is new, red is old):

And here is the complete board with new traces also in green (you don't see removals here):

As can be seen, changes are very subtle, but on the other hand, necessary.

Now, I have to wait few weeks before my order is produced and delivered. Then it will for sure take few more days to solder it properly. I hope this time will be better, as last time I didn't have enough soldering experience to do it right on first attempt, what finally took me couple of evenings to complete. In the meantime, maybe I will try to do some software development, as my plan was to boot CM4 with PXE, or something similar, so I will not need Micro SD card anymore. And this would be good improvement in the direction of setting up everything automatically for Kubernetes.

I also developed BOM and CPL files for SMT assembly a bit further, so in theory it is now possible to order assembly of all 3 ICs present on board - means ESD protection of USB, PMIC for SD card and LED buffer. I even attempted to do so for connectors, too, but with that I failed for now, so this is an exercise for the future due to wrong middle point on my custom footprints. During playing with that I noticed that RT9742CGJ5F (mentioned PMIC) is at no stock whatsoever. Even more disturbing is the fact that there is only one shop I could find, where they claimed to have some and with other Richtek-made PMICs it is not much better, or acutally all of them are at zero stock on LCSC. Fortunately, I still have 9 of the right ones, so for myself it is not a problem, but I can imagine someone trying to reproduce my design and failing to do so due to that. Also this would make any attempt of mass production impossible for the moment, unless redesigned to use different part. It is not a surprise for me, as IC shortage was first heard in context of nothing else, but PMICs.

PS. If you know any PMIC that can be used instead of the one mentioned and which I can easily source on LCSC, or any other non-Aliexpress-or-Taobao place, let me know. I can consider using it instead of the now unobtanium Richtek.

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