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I'm making a PCB that has a 100pin QFP pacakge microcontroller. After gaining experience with the UV photoresist process and reliably making the small, 0.5mm traces, I have a new problem: the copper traces are delaminating when soldering them. Specifically, they get stuck to the solder tip and peel off the board.

It seems this mostly happen when a large part of the trace is heated simultaneously and kept heated for > 1s. I cleaned my solder tip repeatedly to prevent a big blob of solder from accumulating, but that didn't completely make the problem go away. The boards I'm using have a copper thickness of 1 oz.

I'm also using a Weller WPS18MP, which has a tip temperature of ~900F. Is that too high?

Is there anything I can do besides using sheer soldering discipline?

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  • \$\begingroup\$ 900F is definitely too much. You should keep it under 700F. \$\endgroup\$ – venny Sep 7 '14 at 14:45
  • \$\begingroup\$ 900f ok, do it all the time, but you will smoke traces if you aren't careful. \$\endgroup\$ – Erik Friesen Sep 7 '14 at 22:29
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    \$\begingroup\$ Are you using a flux pen or similar? For reference of those of us living in the metric world, 900°F is about 480°C, and 700°F is ~370°C. \$\endgroup\$ – naught101 Sep 8 '14 at 1:11
  • \$\begingroup\$ How come nobody has said anything about using solder mask? Would that hold the traces in place? And I would only need a little, instead of covering the whole board. \$\endgroup\$ – Yale Zhang Sep 8 '14 at 5:41
  • \$\begingroup\$ @YaleZhang Solder mask will certainly help hold the traces in place, but it's not suited to home board fabrication. If you have professionally fabricated boards with solder mask, you would find it much harder to get the traces off. \$\endgroup\$ – Adam Davis Sep 8 '14 at 14:08
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This is a sign of too much heat in the copper strip. That you're seeing this with thin traces suggests your soldering iron temperature is too high, you're using too large a tip, or your holding it on there for too long.

Without more information, I can only suggest my parameters for your situation:

700F temperature controlled iron

Small conical tip (1mm or smaller at the tip)

Lots of flux

Small diameter solder, or paste

You should be able to solder a SMT joint in a second or two without lifting. You shouldn't keep reheating the same trace. If you must use solder wick for rework or blob removal, try to keep the iron on the wick pressing into the joint for under 15 seconds.

If after all this you still experience the problem, you might consider asking your pcb supplier about it. Poor quality copper adhesive or fabrication practices could be the culprit as well.

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  • \$\begingroup\$ +1 - It was 25 years ago, but I worked with an engineer who swore up and down that using a temperature-controlled Weller station with a standard tip set to 698F was the only way to go. I "turned pro" on that setup, so I'm biased, but it's served me well. \$\endgroup\$ – Wesley Long Sep 7 '14 at 23:46
  • \$\begingroup\$ I had a feeling the temperature was too high, especially since I'm using 60/40 leaded solder. I was afraid of having to get a bulky soldering station if I wanted temperature control. But it looks like there are temperature controlled irons built into the handle! Like the Hakko FX-600: hakko.com/english/products/hakko_fx600.html \$\endgroup\$ – Yale Zhang Sep 8 '14 at 0:39
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One way which works well, is to use a different soldering technique. I use a small 'toaster oven' (cost about 20GBP) solder paste in a syringe, and a lighted magnifying glass. I also have a multi-meter with a temperature probe to monitor the process, but in reality a small torch, shone into the oven gives very good feedback. Look for all the solder paste to 'go shiny', and its done.

The toaster oven needs to be at least 1.3kW. Folks I know have failed with attempts to use a less powerful toaster oven. Other people use an 'electric skillet', or pizza oven with very good results.

Apply the solder paste in a very thin line across the PCB pads for the QFP, place the chip, and bake it in the oven. Providing there is very little excess solder, capillary action 'sucks' the solder under the pads, and there is very little bridging to clean up. Use fine (e.g. 1mm or narrower) to remove solder bridges between pins. We have done LQFP64, 0.5mm pitch, this way.

We mostly use manufactured PCBs, but people have used this successfully on DIY FR4 PCBs.

There are a lot of videos on the details of the process. Maybe start at Sparkfun, which had some videos.

Edit:
A couple of techniques which improve the success rate are:

  • Use a fine 'needle' on the syringe. When the hole is fine, it is much less likely you'll apply too much solder. Several vendors sell fine plastic nozzles which can be cut to make a hole to suit the job.
  • Don't panic about making solder bridges. Fine solder braid removes it reliably.

I'll add, this technique was taught to me by a local (UK, West Midlands) Secondary School teacher. He started teaching Surface Mount Technology soldering to 13yo's. They had much higher success rate than through hole parts, which tend to have more difficult to diagnose failure modes. Also, the students found it quicker to build boards; with three toaster ovens a class of 24 could all build a board in less than two hours. They are not using such fine technology as QFP though, typically small PIC's.

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  • \$\begingroup\$ For fine-pitch surface-mount components, solder flux and solder paste really are the way to go... though it does take some practice to learn to dispense the right amount of flux+solder+heat. Soldering a whole row of pins instead of working on each pin individually. \$\endgroup\$ – MarkU Sep 7 '14 at 18:51
  • \$\begingroup\$ Sorry about not taking your answer, but I have used a hot air gun & solder paste for soldering leadless LEDs before. But I'm just afraid of over applying solder paste and causing bridges. I'll try it in the future though. \$\endgroup\$ – Yale Zhang Sep 8 '14 at 0:44
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Yes - 900 degree Fahrenheit is way too hot. Your tip-temperature should be between 650 and 750 degree Fahrenheit for lead-free solder.

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    \$\begingroup\$ +1 I agree, 900 is a bit on the hot side. You can get away with it if you're fast and controlled with the tip, but if not you'll pull the traces off :) \$\endgroup\$ – Morten Jensen Sep 7 '14 at 22:48

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