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Does the solder pattern on these huge pads indicate what might be going on ?

The parts would not work about 80% - 90% of the time. I am trying to follow the ramp_up-soak-reflow-ramp_down curve to the best of my ability. Using a standard 2-speeds hot air gun with no temperature control and set on the second speed as the first speed won't melt the solder. Using various quantities of leaded solder paste (and sometime a bit of flux if I find the paste too dry).

My previous design has had the landmark pad dimensions almost the same as the actual component feet dimensions - caused a great deal of solder bridging, but with these extra long pads no more shorts. However, they still won't solder fine...

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kellogs
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    Are you using leaded or lead-free solder? Are you using flux? – vini_i Jan 27 '19 at 14:06
  • Yeah. You are using inadequate hand tools and trying to follow a process intended for fully automated machinery. Switch to a manual process and achieve 100% reliability: use a soldering iron and hand solder the blipping things. – JRE Jan 27 '19 at 14:21
  • @vini_i Using various quantities of solder paste (and sometime a bit of flux if I find the paste too dry). – kellogs Jan 27 '19 at 15:13
  • @JRE Ehm.. this is an LGA package. How would I iron it ? – kellogs Jan 27 '19 at 15:14
  • What part is it? – vini_i Jan 27 '19 at 15:29
  • All of the parts I can see appear to have standard pins. – JRE Jan 27 '19 at 16:21
  • Replace the heat gun with a proper hot air tool, they cost less than the frustration already spent. Next buy a stereo zoom microscope. With those large unmasked pads you can do manual cleanup with an iron, though you should probably reduce the unmaskee area as it seems to be soaking up a huge amount of solder. Finally you'll need to inspect the inside corner between package and board under high power while tilting the board at an angle to see in the crevice. Video systems tend not to have the dynamic range, and a non stereo system will conceal some flaws that are obvious binocularly. – Chris Stratton Jan 27 '19 at 16:27
  • @ChrisStratton can't I do away without those investments ? A hot air tool would bring only that temperature setting more to what I have, and the temperature on the part / on its pads is not the one set up on the tool anyway. If I am not totally off, the problem is that some part pads do not get soldered to the footprint pads, so no need for microscope either. I just need to perfect my technique, not without some mentoring though :) – kellogs Jan 27 '19 at 17:34
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    If you must solder these kind of components with hand tools, check out Louis Rossman's videos on youtube. He very frequently solders extremely difficult stuff with a heat gun, all on video. It's really easy to popcorn and scorch stuff with a heat gun though, so use an iron for everything you can. When you're thinking about tool investments though, consider that they often pay for themselves and crappy-but-better-than-nothing options exist for hobbyists. I have a $20 digital oscilloscope. I mean it's crappy, but someone from the 80s or 90s would get quite excited over it. – K H Jan 27 '19 at 20:03
  • Heat guns are for stripping paint, hot air tools are for surface mount soldering - it's like trying to sew with a boat hook. You haven't said much about your stencil design but as already explained your pad design is *wrong* and will steal solder. You need magnification to understand the failure. If you want to avoid this, then *don't use leadless parts*. – Chris Stratton Jan 27 '19 at 21:55
  • IC2 looks like a QFN part, not LGA. LGA is a package typically used with a pinned socket, e.g. for Intel CPUs. –  Jan 27 '19 at 22:03

2 Answers2

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First of all, if you are using lead-free solder, it sucks, if possible use leaded solder.

There are two categories of soldering. With interaction and without. Interaction refers to the necessity of having to touch the chip in any way during soldering.

The only way to solder a chip like this without interaction is to use a stencil and solder paste. The stencil dispenses the correct amount of paste. Most importantly the paste is even on all pads. It also almost eliminates solder bridging because the stencil separates the paste on the pads. It looks like you have a single side load. In a case like that, I like to heat the board from underneath, it keeps little chips from blowing away. If you can't do that, then heating from the top is ok but requires a bit more caution. The even solder paste pays off because the chip sits down evenly.

The way I suspect you are soldering the chip is that you apply a strip of solder paste on each side of the chip and then letting the surface tension pull the solder into place. This type of soldering will require intervention. Dispensing just the correct amount of paste is nearly impossible. The tendency is that too much is put on. This causes shorts that need cleaned up after wards. Also, it's suggested that once the solder paste melts, make sure the chip is positioned and then hold the chip down with tweezers and remove the heat. Hold the chip with the tweezers until the solder cools. This will force out any extra solder from under the chip keeping the chip from being crooked. Finally, use a soldering iron with a small tip or knife tip to clean up any solder shorts. Be sure to add flux before removing the shorts.

Another method is similar to above but tin the pads first instead of using paste. Make sure to add a generous amount of flux before putting the chip down.

vini_i
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Also, it's suggested that once the solder paste melts, make sure the chip is positioned and then hold the chip down with tweezers and remove the heat. Hold the chip with the tweezers until the solder cools. This will force out any extra solder from under the chip keeping the chip from being crooked

That was it!! I have just soldered the fourth chip in a row using this method.

Aside from that I'd like to add some more. For resoldering, or for

Another method is similar to above but tin the pads first instead of using paste. Make sure to add a generous amount of flux before putting the chip down.

I have made it work as well, but the key here is to add tacky, not liquid flux. The liquid flux will just start boiling before the solder starts melting and will displace the part. Using a stencil only solves half of the problem I think. It requires precise component placing afterwards in order to take advantage of those perfectly dosed & placed solder blobs. Way too hard for just my hand to do the job. I have also tried the 'heating from beneath' approach last year with terrible results. Due to the 'benath' being mostly a copper plane and to the 1.6 mm thickness of the FR-4 board I had to put out insane amounts of heat which has just caused the soldered parts to fail over a short period of time. I would not recommend it, at least not for sensitive parts.

Another thing - I have made my pads 4mm long, which is kind of overkill for the 2x2 mm LGA chip that I am seating on top of these pads. They surely work fine, but I think they could be reduced to 2 mm length without issues. They got me almost completely free of the solder bridging issue that I have had with my previous design where the pads on the PCB were about the same size as the pads underneath the part.

Oh, and yes, I have been using leaded solder paste all along.

kellogs
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