I am trying to decide on a pinout methodology for a module that will be assembled board to board on a bigger PCB.

considering the pros and cons of LGA vs. castellations(cut half vias), i was thinking of combining the two - using a small amount of castellation pins and a bigger amount of LGA's, so the user could choose if to use only some features (with easier assembly line) with only the castellations or to use the full features by connecting both castellations / LGA's.

is this mechanically possible? does the assembly process of one negates the other in some way? (height, heat, etc..)

I would very much like some opinions/suggestions on this matter.

  • \$\begingroup\$ Things like this do exist, for example some SOM compute modules do it, presumably the idea is that customers with simpler needs have an easy time working with the castellations, while those who need to leverage more of the I/O can design a more complex board using the LGA pads. \$\endgroup\$ – Chris Stratton Dec 27 '18 at 15:26

Thank you for posting this question. Mechanically, there is nothing that will stop you from using both assembly methods, as long as the receiving PCB, to which you assemble the module caters for this. What I mean here is that you can make a receiving PCB with either method, or even both, but you will have to remember that if you use castellated pads and not LGA, you will still have the LGA pads exposed under the module, and therefore you will risk shorts if you have any copper exposed on the receiving board, for example vias.

Electrically, you will have to keep in mind that you will be adding a stub to all the lines you bring out, which means that the integrity of hi-speed signals will be compromised.

There is nothing stopping you from mixing the two technologies.

One thing that is worth mentioning, is that to use LGA pads, you will have to have a controlled assembly process, which will use solder paste masks and some form of solder reflow. Castellated pads are easier to hand assemble, even by an untrained user, and is easier to inspect, however, it can cost a little bit more to manufacture.

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  • \$\begingroup\$ thank you, i am aware that making extra connections will create stubs, although in my case it is irrelevant sins I want to use the LGA's as extra connections, so recipient boards that will need those connections can add them and if not then the recipient board will be without the LGA pads and will have an easier assembly process. thank you again for this comment, it helps a lot also to know that there are products that do this. Roy. \$\endgroup\$ – Roy Barak Dec 29 '18 at 12:11

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