Equipment for reliably soldering QFN, BGA etc

[AndyC_772]

Hi all

A job has come up for which I have a couple of options; either (a) pay to have a few prototype PCBs assembled by a contract manufacturer, or (b) buy the parts and do it myself. Since the cost of (a) has turned out to be higher than expected, (b) looks viable even if I have to buy some new tools.

(Yay, new tools!)

The board I'm looking to assemble has a few small QFN parts on it and one coarse pitch (1.27mm) BGA, so I'll need appropriate equipment to solder these components reliably. Whatever I use needs to be easily available commercially (no DIY modified toasters and the like).

There are a few suspiciously cheap hot air rework stations available...

https://www.expondo.co.uk/stamos-soldering-soldering-station-with-pre-heating-plate-and-2-clamps-6109?c=1303

...or...

http://www.ebay.co.uk/itm/AOYUE-Int866-3in1-Rework-Station-lead-free-Hot-Air-Soldering-Station-/332329992863

...or...

http://www.circuitspecialists.eu/csi-853b-plus-digital-preheater-station-with-hot-air-rework-option/

I'm sure these would be ideal, provided of course that the one I choose is of adequate quality to actually do the job. For comparison, a JBC JTSE-2A is about £1400 and doesn't include the pre-heater.

Any other suggestions please? This is a commercial job, so I can afford a tool which is right, just not one which is OTT.

[nctnico]

I think a hotplate is a good option for BGA. There are BGA rework kits which consist of a stick-on paste mask so you can apply solder paste locally. For QFN you can use a regular iron if you let the pads extend beyond the case and put vias in the thermal pad so you can solder the reverse side using hot-air.

[nanofrog]

For an excellent hot air station that won't cost a small fortune, I'd recommend the Quick 861DE (link = UK seller).

Depending on the # of layers, you may need to get a preheater of some sort as well. Personally I went with a cheap hot air unit as I don't use it all that often (board held in a Panavise 315 or 376), but even an electric skillet would work.

[sn4k3]

For an excellent hot air station that won't cost a small fortune, I'd recommend the Quick 861DE (link = UK seller).

Depending on the # of layers, you may need to get a preheater of some sort as well. Personally I went with a cheap hot air unit as I don't use it all that often (board held in a Panavise 315 or 376), but even an electric skillet would work.

+1
Follow this and get away from cheap crap 
Quick 861 all the way

I have the Quick 861DW and a IR bottom plate when needed

[AndyC_772]

Thanks everyone. Today I managed to track down a discontinued, but brand new unused JBC hot air station at a substantial discount off the price of a new current model, so that's on order and should be here early next week.

Still looking around for a pre-heater and a suitable stand.

[cstratton]

For heating, unless you have extreme heatsinking issues through the board, a simple wand type hot air skillfully applied can do a lot.  For prototypes I don't use an oven, nor do I want to scorch boards on a hot plate - I just do it with the wand while watching under the microscope for any issues so that I can immediately correct them.

Various preheat from below solutions may make sense as well.

That's all fine for QFNs and anything with perimeter pins.  The bigger problem with BGAs is placement.  At course pitch you maybe be able to do it visually in reference to board features you trust, especially if you have something like an auxiliary microscope webcam you could aim in from the sides to double check.  But the proper tool is one which has dual imagers or a prism, so that you are looking "up" at the chip on a vacum pickup slide and "down" at the pads on the board and aligning the two superimposed images.   When satisfied you move the imager out from in between the two and lower the head with the chip down to the board.

Here's Louis Rossmann doing it manually:

https://youtu.be/4Y2y6zpupd4?t=30m41s

here he is (coaching someone) to do it with a basic implementation of a proper superimposed image BGA placement machine:

https://youtu.be/Shn7LdIrViQ?t=2m55s

All that said, one of the biggest problems to hand-build boards isn't technical, but just the time and tedium of pulling out all the little parts and placing them one by one in the stenciled paste.  It's not the chips, it's the passives.  Definitely get a vacum pickup, after discovering the Aoyue was absurdly oversized the next time around I converted an aquarium pump.

[Howardlong]

Does anyone have any recommendations for BGA inspection? My assembler predominantly uses a tiny mirror periscope thing, you can see right down a row at a time, and adjust the focus appropriately as the depth of field is tiny.

Does anyone have any suggestions on how one would jury rig something like this without spending a fortune?

[nanofrog]

Does anyone have any recommendations for BGA inspection? My assembler predominantly uses a tiny mirror periscope thing, you can see right down a row at a time, and adjust the focus appropriately as the depth of field is tiny.

Does anyone have any suggestions on how one would jury rig something like this without spending a fortune?

I've looked at the prices of what you describe... (ouch  ), and it gave me an idea... perhaps a simple prism in conjunction with your microscope would do the trick? 

[cstratton]

I'll often just raise my microscope and hold the board in my hand at an angle to try to look at the first row under the edge.

I think if I were trying to place a BGA manually before soldering it, I'd try one of the narrow USB microscope-webcams aimed in at an angle while keeping the board level under my main scope.

[Howardlong]

The problem with that approach is that it assumes no other parts are in the way, and the board will fit and focus within the microscope’s working distance.

Imwas thinking along the lines of a tiny first surface mirror on the end of a handle of some sort, I might need to experiment.

[nanofrog]

The problem with that approach is that it assumes no other parts are in the way, and the board will fit and focus within the microscope’s working distance.

I was thinking along the lines of a tiny first surface mirror on the end of a handle of some sort, I might need to experiment.

True*, but so do the simple mirror devices I've seen (nothing more than a mirror and a bit of wire to hold the angle on the PCB itself (60 degrees?).

The rotating bit on the bottom side of the Barlow/camera is nice (i.e. replicate by gluing a couple of mirrors to a 48mm light ring), but I don't think it's an absolute necessity. I'd likely suck at gluing the mirrors in the correct positions anyway, so would have to rely on moving the head & board regardless (though trying to make one wouldn't be expensive).

* I was thinking of soldering to a bare board prior to any other components. Completely ignored repair/rework situations of course ... 

This is what I'm thinking of:

[Howardlong]

Me too, I guess I need to get hold of the right surface. I was thinking about dental mirrors, but they are typically round and have a bezel, perhaps it’s a simple as grinding them a flat edge assuming there’s no glass, it’s just a mirrored surface.

[nanofrog]

Me too, I guess I need to get hold of the right surface. I was thinking about dental mirrors, but they are typically round and have a bezel, perhaps it’s a simple as grinding them a flat edge assuming there’s no glass, it’s just a mirrored surface.

FWIW, my dental mirror is glass. And as it's a size 5 (25mm), it may be a bit small for some of the more interesting IC's.

A molybdenum mirror is interesting (Mo mirror), but it would have to have the edges ground flat. It's also a bit thick at 3mm, so I'm uncertain if they're truly fit for purpose. So stainless steel may win out as being the easiest way to go.

[Stavos122]

Relying on PCB silkscreen markings for alignment should work for coarse pitch BGA like 1.27mm.  Considering that even if you are off by 25% off center, the part will self align during reflow.  Micro mirror may help too but you have to take into account proximity of the adjacent components.  It may be difficult to get a mirror in a tightly packed board with passives around a BGA part.  When dealing with fine pitch BGA or QFN parts that have flat pads and minimal board clearance, the tool to use is a rework station with optics like this AT-GDP SMD BGA rework station https://www.atco-us.com/products/at-gdp-smd-bga-placement-rework-stations .