Platform forum

Termination resistors for DDR4 designs under BGA

ntstuart , 05-11-2017, 11:44 AM
I am asking my company to let me use 0201 size termination resistors and decoupling caps under my BGA since we decided to go with through hole dog bones.
They are suggesting via-in-pad as an alternative which makes no sense, it's a question of space. Any thoughts on this? Could it even be done? I don't think it's possible.
robertferanec , 05-11-2017, 05:44 PM
Normally we place termination resistors on DDR3 at the end of the tracks (not under CPU, but after the last memory chip). I have not done design for DDR4, but be sure, they do not specify where the termination resistors should be placed. Let us know then where they should be placed. I am interested to know. Thanks.
ntstuart , 05-11-2017, 07:57 PM
These particular termination resistors are from the CSI2 circuitry for a DS90UB960 FPDLink, and must be under the FPGA where they connect. You are correct, the DDR4 signals terminate at the last memory chip. I didn't phrase the topic correctly. In any case, I proved to them that we needed 0201's to route them under the BGA. I was baffled that they even thought 60 0402's could be broken out of 0402's in that location. We finally got your program Robert, I start this weekend!
mairomaster , 05-12-2017, 01:37 AM
Here is my reply from the duplicate topic (that one should be delete not to cause any confusion):

I would always go with 0201 rather than using micro vias in pads. Your manufacturer will be much happier and your assembler shouldn't have problems with 0201 if it's a decent one. Using uvias in pads especially tends to be problematic and expensive.

And just to make it clear, if you are using vias in pads, they definately need to be micro vias - it doesn't work with TH ones, the holes are too big. That could save space on the bottom layers, as the micro via pads are much smaller compared to the TH via ones. The next alternative is using uvias, but not placing them in the pads. This is a bit better in terms of reliability.

What is the reasoning of your company?
robertferanec , 05-12-2017, 12:04 PM
I completely agree with @mairomaster. We have never had any issues with 0201 (all proper assembly houses are capable to fit these), but in past we have had problems with VIAs in pad and since then I never use them.
ntstuart , 05-12-2017, 05:33 PM
I completely agree with both of you, I just had to convince them. They haven't done any high speed designs before, but I have done many. I am doing the entire design with through via's because I do not see a need for them. Also, no need for via-in-pad, it's not as complex as what I've done at other companies. They are being ultra-conservative because our industry is automotive, that's their reasoning, but HDI has it's own set of common rules which are typical across all industries. Thanks guys!
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