Seek for anything can fix the position of power module on PCB

[R_ZH]

Hello everyone,

I'm working on a PCB design in altium and have a very tricky question. I have a MOSFET power module which has a rectangular baseplate and need to put it on the PCB surface. And only a small area of the baseplate will be soldered to a pad, not the complete area. But I want to fix the position of the power module when I put it on the PCB to avoid any measurement error. I think there should be some components like position fixer, but I don't know what exactly the name is. Does anybody know the name or other method to fix this problem? Any help would be appreciated.

Best regards and thanks in advance!
Runze

[ajb]

What do you mean by "fix the position"?  Do you mean you need to mechanically connect the PCB and the module?  If so, the kind of part you'd need will depend on the module you're using.  Or do you mean setting the location of a 3D model of the module in the PCB file? Or creating reference geometry on the PCB so you can account for the size of the module? Or something else?

[tszaboo]

Post the part number and I'm sure we can recommend something to secure it mechanically.

[R_ZH]

I think it's setting the location of the 3D model of the power module on PCB. I'm considering using something raised like a fence which has the same size as the baseplate to surround the power module, to make sure everytime I place it on the PCB, the position is always the same. I'm using FF45MR12W1M1_B11 MOSFET power module from Infineon. Here is a bottom view of the footprint, only a small part of baseplate surface will be electrically connected to the pad by soldering. The other part should be placed on the PCB in the way I mention before.

[PlainName]

The pins are on the other side, aren't they? So the holes for those should orient the part. Or are you saying you're attaching a PCB to this non-pin side? In that case, can you not use the holes in the wings for positioning?

Edit: attached the data sheets to save anyone else looking it up.

[R_ZH]

Yes, the picture is a bottom view and the PINs are on the other side. I will design a top PCB to fix the position of the PINs. Yes, I will attach a bottom PCB to the non-PIN side, and get a multi-board PCB. In the  lab, the white cap of this power module will be removed and only the baseplate (brown part in the picture) and PINs are left, then soldered to the pad (yellow part),. And this rectangular surface will be directly placed on the bottom PCB. There is no solder for the surface, therefore not fixed. So I'm looking for something can fix the position of this baseplate surface on the PCB.

[ajb]

Okay, so sounds like you're looking for a way to manage the alignment of the physical part when you assemble the real PCB? 

To start with, I would create a PCB footprint for the module, including the 3D model and some 2D reference geometry (on the 3D Body layer or just on an additional mechanical layer).  You can include a silkscreen outline to visually align the part to when assembling, depending on the overall assembly style that might be good enough.  Make sure the silkscreen marks are very close to the edges of the module, if the edges of the module land on the silkscreen lines that will probably be best. 

If that's not good enough, you could add holes to the PCB footprint for dowel pins to align the module.  You'd want three pins: two on one side of the module, and one on an adjacent side, near three of the four corners.  Sketch attached.  Push the module down against the two pins and then left against the third pin, and that will get pretty good alignment of the module on the left and bottom edges.  Once the module is fixed in place, you can pull the pins out.  The holes should be a fairly close fit to the pins.  Or, you could build an alignment jig that holds the module in place while soldering.  Jig could be 3D printed, laser cut, built up from PCB scraps, or whatever.

I'm a little skeptical of soldering one corner of a module like this.  Unless the edges of the module's baseplate are solderable you'll need a layer of solder between the module baseplate and the copper of the PCB, which could make the module sit unevenly, or even stress the PCB if the module is clamped down tightly.  If you keep the solder mask intact under the rest of the baseplate that might give enough of a gap between the module and the pad to allow for a bit of solder.  Adding silkscreen to the area under the module would increase that gap and might be better.  Both of these would reduce increase the thermal resistance between the module and the PCB though, which might be an issue for you?

[PlainName]

In the  lab, the white cap of this power module will be removed and only the baseplate (brown part in the picture) and PINs are left, then soldered to the pad (yellow part),. And this rectangular surface will be directly placed on the bottom PCB.

OK, that explains the problem 

[Batang]

The FF45MR12W1M1_B11 device is End of Life

Correct me if I am wrong.

[R_ZH]

Thanks a lot for your answer!

Yes, I'm looking for a method to align the power module on the PCB. From your advice, I think silkscreen could be better since adding mechanical components like dowel PINs and alignment jig may induce parasitic inductance.

Soldering one corner is also my concern. In real PCB, the pad surface is a little lower than the PCB surface. If put the the PM on the pad, there is a air gap between the baseplate and the pad surface. Put only a corner of baseplate over there on the pad surface and I solder on the remaining part of pad surface, after melting, the liquid will flow into the gap between the corner of baseplate and its covered pad surface. I do this just want to avoid inducing any extra parasitic components.

[R_ZH]

Sorry, what do you mean by 'end of life'? Is it discontinued or out of sell?

[Batang]

Scheduled for obsolescence and will be discontinued by the manufacturer - Mouser Electronics

New generation M1H product FF33MR12W1M1H_B11 coming soon - Infinion

[R_ZH]

I don't know this, maybe you are right. I work on this just because this PM is one of the investigated SiC PM in our research group.

[ajb]

I think silkscreen could be better since adding mechanical components like dowel PINs and alignment jig may induce parasitic inductance.

The pins and/or jig would only need to be in place while the module is being soldered. Once that's done they can be removed.

Soldering one corner is also my concern. In real PCB, the pad surface is a little lower than the PCB surface. If put the the PM on the pad, there is a air gap between the baseplate and the pad surface. Put only a corner of baseplate over there on the pad surface and I solder on the remaining part of pad surface, after melting, the liquid will flow into the gap between the corner of baseplate and its covered pad surface. I do this just want to avoid inducing any extra parasitic components.

  Parasitics to what?  An additional soldering pad at another corner of the module likely won't make any difference--the geometry of the baseplate is going to be much more significant in terms of any sort of capacitive coupling to anything else nearby.  It will be tricky to solder though, since you will need to get the baseplate of the module hot enough for the solder to wet to it, and that will be tricky if you can't make direct contact between the soldering iron and the baseplate.

[R_ZH]

Ah, OK. I get it.

I mean if I solder the entire baseplate to the PCB, that is a big surface, it has many soldering point after the PM is completely fixed on the PCB. Next time I do this again, the soldering point will be different. This may induce some errors, maybe the parasitic components induced by different soldering point. I'm not sure but just thinking. I use this PCB to measure the stray capacitance inside the PM, which is on the level of pF. So any potential error must be controlled to minimum and try to avoid any change to the experiment everytime. Even a very small error can make the results meaningless.

Yes, it is hard only to solder a corner of the PM. I even don't know if that is possible in the Lab. So what is your advice? Any good idea to do that?