Tips for soldering this thing?

[MacAttak]

SMD Microphone - ADMP421 - http://www.analog.com/static/imported-files/data_sheets/ADMP421.pdf

This is being used on a small 2-layer PCB (not hand-made). I'm using decent (fresh) leaded solder paste, and a hot air station to reflow it (the same WEP/ATTEN 858D that Dave played with a while back). Simple Weller iron for touch-ups. I don't have a solder stencil, I've just been relying on dabbing the paste on the pads carefully and relying on the surface tension and soldermask to tidy it up during reflow.

This is what the footprint looks like (although the silkscreen layer didn't get printed so placement isn't easy at all without a p&p robot doing it ):



I didn't have any trouble with the rest of the components on this board - 603 caps and resistors, and no problems with resistor arrays or the SOT transistors, or the RTC IC. All of those were fairly painless and I only had a couple solder bridges to repair on the arrays.

But this microphone is being a royal PITA. The solder seemed to wet and pull the device into place, but it failed testing afterwards. I ended up using the heat gun to pop it back off, and it looks like maybe the VDD and/or L/R pads didn't reflow. I don't trust the device to be undamaged at this point, so I'm ordering a replacement. They are $4 each though, so I don't want to keep chewing through them by failing the reflow again (and there is also the danger of board damage with these tiny pads). Lastly, there is the concern about solder flowing into the microphone port... the ring around the port must solder correctly, but without any flowing into the hole itself

Any tips for getting this thing soldered correctly?

I'm thinking of making a poor-man's solder stencil using a paper stencil (if I can manage the precision needed for that). But other than that, I'm not sure what other tricks I could pull.

I could also ditch this part altogether and replace it with an external condensor microphone circuit, which would also require some changes in the FPGA code that this board hooks up to... but I would rather get the original designed digital microphone part working if I can.

[MacAttak]

Oh - I'm sure someone will ask anyways, but this is the board I am populating: http://embeddedmicro.com/development-boards/clock-visualizer-shield.html

Normally, these are sold fully assembled. But I had the great idea that I would like to practice my assembly skills so I convinced them to sell me one unassembled Hey, it sounded like a good idea at the time.

All of the other functions of the board work perfectly, it's only the mic that is giving me fits.

[lgbeno]

Wow that package looks brutal!

Are you trying solder paste and oven reflow at all?  Looks to me like you just need a small dot of paste, too much and it will bridge.  I would also skip the outer pad ring at first and fill it in with a soldering iron after you know the 4 inner pads are good.

[cwz]

You might try the following:

Put some solder down on the pads and reflow it with the iron.  You want to keep a relatively uniform amount of solder on each pad, but it's not super critical.  Put some tacky flux down on the pads and heat it with the hot air.  You wan the flux to go liquid, but mostly to drive off the lower temperature solvents.  You don't want to boil away all of the flux, but rather to end with a more sticky flux residue.  If you're concerned about heat damage to the part, try placing a bit of Kapton tape over top of the part.  This will help mitigate the effect of the hot air a bit, and you can easily remove it when you're done.

Place the component down, and run a small bead of flux around the outer edges.  (I'm assuming you're using a pencil tip or some kind of nozzle to focus the hot air to a smaller stream)  Move the hot air in a circle around the part -- you want the flux to go liquid and move like a wave around the outer edges of the part.  This and the moving air will help break the surface tension of the solder and cause it to reflow.  If you have a microscope, you should be able to see the solder go liquid as it hits reflow temp.

Best of luck

[MacAttak]

Wow that package looks brutal!

Are you trying solder paste and oven reflow at all?  Looks to me like you just need a small dot of paste, too much and it will bridge.  I would also skip the outer pad ring at first and fill it in with a soldering iron after you know the 4 inner pads are good.

I have no oven, but yes I am using paste. All of the pads (including the outer ground ring and frame) are inaccessible once it is on the board - no "fill in" is possible once it is placed.

[MacAttak]

You might try the following:

Put some solder down on the pads and reflow it with the iron.  You want to keep a relatively uniform amount of solder on each pad, but it's not super critical.  Put some tacky flux down on the pads and heat it with the hot air.  You wan the flux to go liquid, but mostly to drive off the lower temperature solvents.  You don't want to boil away all of the flux, but rather to end with a more sticky flux residue.  If you're concerned about heat damage to the part, try placing a bit of Kapton tape over top of the part.  This will help mitigate the effect of the hot air a bit, and you can easily remove it when you're done.

Place the component down, and run a small bead of flux around the outer edges.  (I'm assuming you're using a pencil tip or some kind of nozzle to focus the hot air to a smaller stream)  Move the hot air in a circle around the part -- you want the flux to go liquid and move like a wave around the outer edges of the part.  This and the moving air will help break the surface tension of the solder and cause it to reflow.  If you have a microscope, you should be able to see the solder go liquid as it hits reflow temp.

Best of luck

Thanks, I'll give the kapton a shot. I'll also try to pre-flow the pads and get them tacky with flux as you suggest.

Yes, I have a small nozzle on the hot air tool.

When I made the first attempt I was certain that it reflowed (I could see the part suck down to the board) - but it seems to have not bonded on one or two of those inner pads. They were clean of solder when I reheated and popped the mic back off.

[IanB]

One thought that occurred to me was to try reflowing using a hotplate, heating the board from underneath with the part on top. Some people have successfully done this using a cast iron griddle pan or similar. The advantage would be that the whole board area under the part will reach reflow temperature and the part itself will not be subject to excessive heat from above.

[nukie]

Agree with Ian, preheat with hotplate below at 120-150C then hot air from above, this will reduce thermal load on your hotair iron and shorten the high heat times, thus less damage to the parts.

[MacAttak]

I had considered that - but if I were to heat the whole board then at the very least I would need to remove a few heat sensitive parts - such as the pair of RGB matrices and maybe the through-hole momentary pushbuttons and buzzer. Since I don't have a proper solder iron with vacuum for rework, it wouldn't be very easy to remove those parts without damaging them.

I'll give the kapton+flux a shot, and if that doesn't work then I might try building up my own microphone circuit on a tiny add-on board and hack it into the Verilog code.

In some ways, I would probably learn more from doing the latter anyways. And that's the whole point of taking on this challenge in the first place.

[IanB]

If assembling the board from scratch you should do the hardest and most complicated parts like the microphone first, then fill in around it with the cheaper and simpler parts. I guess that only works if you have a second board. But perhaps you should anyway? For anything new and unfamiliar you should do a practice run to iron out the kinks, and then do the real thing afterwards.

[MacAttak]

That's exactly what I did - this was the first part soldered, and the mic appeared to solder correctly. I just simply didn't have an easy way to test it until the rest of the board was assembled.

I suppose that I could have possibly assembled a test rig of some type, after spending time with the part's datasheet to figure out how to drive it directly and read a signal. But it occurred to me - what if it didn't work in my test rig? I wouldn't be confident that my test was reliable, and I would still be guessing if the part was really bad or just improperly soldered. Basically, the same situation I am in now, but now the rest of the board is my "test rig" and I can confirm that there is a problem isolated in that one device.

This is just a development/learning board (not my design) with ultimately not that many parts on it; I didn't have a reason to obtain a backup. Plus, even without a working microphone, the board is still useful as something to explore with as I learn HDL.

[lgbeno]

By the way when I say oven, I mean a $30 toaster over, probably even cheaper if you buy one at a second hand store.

[digsys]

I bet you won't make THAT mistake again :-)
I've been putting off placing my first BGA (stoopid IC is not available in any other package). Not looking forward to going through similar to you :-)

[cwz]

I bet you won't make THAT mistake again :-)
I've been putting off placing my first BGA (stoopid IC is not available in any other package). Not looking forward to going through similar to you :-)

Don't be put off -- BGAs are much easier that you might think.  You can use the same technique I mentioned above.  Along with most other SMT parts, the surface tension of the solder will by and large cause them to self align when they hit reflow.  If you get the placement generally correct, you will probably have success.  When I say this, I'm assuming you're not trying to do a < 0.5mm pitch part and/or not a 1000+ pin large package for your first attempt 

[MacAttak]

By the way when I say oven, I mean a $30 toaster over, probably even cheaper if you buy one at a second hand store.

I figured that's what was meant. No, I don't have one - although it's high on my list of projects to do. The only reason I really haven't done it yet is that space on my workbench is quite limited.

[MacAttak]

I bet you won't make THAT mistake again :-)
I've been putting off placing my first BGA (stoopid IC is not available in any other package). Not looking forward to going through similar to you :-)

Yeah, next time I will make sure to have a plan in place to test the bonding of connections that are hidden from view before I proceed with soldering any of the other parts. In retrospect, this microphone doesn't really seem all that difficult to test assuming it reflows properly. It's just got a GND, VCC, clock, and digital output. And all of those lines eventually end up attached to standard 0.1" header pins. Should be very easy to probe and verify on my scope.