Power connector contact resistance

[luky315]

I have a SMT power connector (custom made) with a 30mm² tinned copper pad. My qustion is: What is (approximately)  the contact resistance from this connector to the PCB pad if a standard lead-free solder is used?

[Siwastaja]

You could calculate it using tin's resistivity and assuming some solder layer thickness between the PCB pad and component pad, like 100µm.

But I'm sure the result will be so small that everything else matters more: specifically, what kind of track/fill pattern are you then using on the PCB?

[luky315]

My problem is that I have no Idea how thick the solder layer is. The PAD on the PCB is massive (40x40mm)with 4 Layer 70µ.

[Siwastaja]

Well, if you use a 100µm thick stencil (my favorite thickness, I prefer it over 120µm now), doesn't that kind-of set a maximum limit on the solder thickness?

In reality, the paste layer has deliberate gaps in it (segmentation) spreading the paste over larger area (reducing thickness), and the flux also gasses and/or flows out of the mixture, so I would expect the end result is significantly thinner than the original paste print thickness. But you can't go wrong if you assume the worst and calculate using stencil thickness.

[Faringdon]

....Taking Rho of unleaded solder as 1.3e-9
With your 30mm^2, and 50um, this gives resistance of some 217 nanoOhms.

AYK, if you have thermal reliefs etc, then they will likely swamp out the resistance effect of the solder fillet.

So the pad is about 5.4mm by 5.4mm ~30mm^2?.....or nearer the 40x40mm (=1600 mm^2) that you later mention?

[luky315]

I've got measurements in a german document which are in the region of 50µm solder thickness, which makes sense, because the solder paste has a lot of volatile components (flux etx.).

[Siwastaja]

BTW, it's hard to guarantee perfect solder fill between the pads, unless you can X-ray the units, you should assume some amount of voids (basically air / trapped gases). So if you calculate at 100µm thickness and 70% of the pad area, you are pretty safe.