Electronics > PCB/EDA/CAD

PCB Edge Traces for QFP


Sorry if this is the wrong sub forum, please move the thread if it is. Also is this has been answered before, please kindly link me to the appropriate thread.

I have not done PCB schematics / layout in a long time, so I have been out of the game for awhile. I do however work with electronics in a daily capacity. I have an interesting requirement for a prototype where we have an existing PCB that has a 176QFP CPU on it and we wish to replace this CPU with another one that is completely different plus a bunch of other prototype circuitry. This means pinouts between CPUs and the prototype circuitry do not match. We do not want to re spin the PCB, because it is very complex... we only want to use it to replace the existing CPU and prototype using a different one.

The QFP176 is very large in PCB area - about 1 sq-in. What I was thinking about was making two "daughter" PCBs. One to act as an interposer that solders directly to the QFP176 footprint. This interposer board would have edge traces that solder to the QFP176 footprint and then have high density connectors in the middle to break out the 176 pin connections. The second PCB would have a mating connector on the bottom and would press fit into the interposer PCB. In this way, the QFP176 on the board and the interposer essentially just acts as an 176 pin I/O connector. The mating board can then be made to any size and with any circuitry that mates with the interposer.

I have done some searches online and I have not found a ton of hits. However, I have found a company that makes a QFP board similar to my idea. They can be found here: https://www.aprilog.com/Corrector_SL.htm

The particular board is found here: https://www.aprilog.com/images/176QFS16-QFS20-SMT-SD.jpg

So.. here is my question. I am looking to make an interposer board with 'edge traces', not castellations. The edge traces that I am looking to make are shown in the linked JPG. Castellations are a well known thing to put on PCB edges, but they are using through holes at a pin pitch larger than what I need. I would like to make a board with edge traces at 0.5mm pin pitch so the board can mate with the QFP176 footprint.

How are edge traces of small pitch (0.5mm) usually done in an EDA tool? Are there any resources or any special needs in order to make them? 

They show boards with 0.5mm pitch castellations on that site:

Either its with castellations, or, you use vias to go from top to bottom layer, then have a blind SMD pad on the bottom layer.
This is common on some SMD modems: https://www.powersystemsdesign.com/images/articles/1643400686-Telit_WE310G4_group.png

I don't think there are "edge traces" anywhere here.


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