148-pin QFN case on 1 mm PCB

[Altera_Forum]

Hello, 

 In its Cyclone 4 family Altera introduced new 148-pin QFN case.   What is the reason to recommend rather thin PCB, <0.8 mm, for this case? Is it PCB thermal expansion? And what can I do if I need 1 or 1.6 mm, e.g. because of edge connector? If I use some laminate other from FR4 for my PCB, could it help?   Best regards,

[Altera_Forum]

Hi, 

 there is an answer in the knowldege base : http://www.altera.com/support/kdb/solutions/rd07052010_123.html If you have a PCB thicker than > 0.8mm you should use a BGA package.  Regards, HJS

[Altera_Forum]

Hi HJS, 

 Thank you for the reference. Unfortunately I can not use a BGA, it's too large. There's absolutely no way to fit it into that PCB. And it is very difficult to make a complicated PCB, 0.8 mm thick in one area, 1.6 in another one. So I'd be happy to know what is the problem exactly, is it just because of different PCB thermal expansion during assembly? If yes, is it reasonable to choose some other material for my PCB, rather than FR4?   Regards

[Altera_Forum]

I'd use the recommended 0.8 mm FR4 with surface-mount connectors.

[Altera_Forum]

The knowledge base explanation isn't particularly clear. But there's no other thinkable reason than thermal expansion in my opinion. It's your decision if you follow the Altera suggestion or ignore it. I think, there's a lot of chip packages that are exposed to similar specific solder joint forces due to thermal expansion.