LAY-27392_C.brd Via Question

[atillametetured]

Greetings,

I know this will be very specific but I am just very cruious about the fact that, In the board mentioned in the title, which is basically Sabre for Smart Devices layout file, for the DDR signals, a via named C008-008TENTED-DDR is used, which is a modified version of C008-008TENTED without the bottom soldermask defined.

Now when one goes to the constraint manager in search for this via, one cant find anywhere this via is assigned to. None. Not even going to "Add Connect" mode on the exact via (C008-008TENTED-DDR) and it doesnt show up in the via drop down menu.

I did go through extracting and searching thoroughly. So can some hardware expert please demistfy the usage of this ninja via?

Best Regards,

Mete

[TheAdmiral · Accepted solution]

Hi Mete,

The basic via is C018-008OPEN, which does not place a solder mask over the exposed via pads.

When placing a via underneath a BGA component to bring a trace up to a pad, it is a good idea to place a solder mask over the via so that the trace and via pad don't wick away the solderball from the component, resulting in a no-connect. The bottom via pad is not covered wtith a solder mask to assist the Assembly House in performing post assembly testing; In particular, a flying probe test that uses needles on top of exposed copper to look for shorts mostly, but can also test for conitinuity from point to point. We label the via with one solder mask as C018-008TENDED

The special via, C018-008TENDED-DDR, has a solder mask over both top and bottom via pads. That is because there is a BGA package on both sides of the via. You don't want to wick your solder on either side, and in any case, the machine perfoming the flying probe testing cannot access this via, so there is no point in leaving it exposed.

The special via, C018-008TENDED-CPU, has only one solder mask, but is different in that the thermal pads are smaller, so that they can be densly packed underneath the processor.

The special via, C018-008CLOSED-CPU, has the smaller thermal pads, but this is the version that has solder masks on both sides, not that there is any BGA on the bottom side of the processor, but the density of passives and vias underneath the processor basically requires a soldermask on both sides.

These are the only C018 vias I see in this design.

Could you please tell me if this question is in response to a specific custome question (and the processor), or if this is just for NXP internal?

Please let me know if you have any additional questions.

Cheers,

Mark

[gusarambula]

Thank, you, Mark!

[atillametetured]

Greetings Mark,

That was a very definitive answer from you, I absolutley have no questions left about the via usage in this board, I thank you for that.

Regarding the aim of the question, we are building a custom board with i.MX6 and taking Sabre as a reference. I do not wish to alter the way most of the structure was built in order to comply with NXP's guidelines. My question was asked in light of this.

I do not have any other questions for now but if I do, I will surely contact you.

Cheers,

Mete