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I have designed a circuit board that has about 20 parts on it, including a surface mount LMV431 Adjustable Zener Shunt Regulator and MMBT2907 PNP transistor.

It appears that every time I solder up a board, the Zener and/or PNP transistor get cooked and do not function properly afterwards. Specifically, the Zener never goes to a high-Z state as it is supposed to (keeps conducting regardless of conditions) and the PNP transistor doesn't shut off (keeps allowing current flow even with zero base current).

Here is my soldering process:

  • Heat up bottom of PCB with hot air rework station set to 150C on maximum air setting for about 2 minutes holding hot air tip at 30° relative to PCB. In last 20 seconds ramp temp up to 280°C
  • Reduce air flow to ~4/8 setting, keeping temp set at 280°C, and immediately begin heating up entire top surface of the PCB, moving the tip constantly at a speed of ~ 8cm/s and tip distance of approx. 1.25”. Do this for 20-30 seconds
  • Reduce tip distance from board to ~0.5” and begin focusing heat in 2 cm diameter swirls on the section of the board nearest the USB connector landing area. Focus heat on capacitors and resistors in this area, and leads of FET. Make sure to revert back to the 8cm/s area heating from the last step on the part of the board that is not being reflowed in order to make sure its temp stays up. Probably reverting back to this every 10 seconds for 1 or 2 seconds is advisable.
  • Blow on 45° angle all around the FET to get the solder underneath to reflow
  • Move reflow target area to the Zener/PNP side of PCB, taking care to keep tip moving fairly fast. Try to avoid aiming tip directly at Zener or PNP, instead trying to blow just to the side of them to get their leads to reflow

According to the Zener datasheet, the maximum temperature during soldering is 260 C:

enter image description here

At this point I'm worried because I can't seem to assemble a working prototype in order to validate the design (need to be 100% sure that the inconsistent go/nogo between the 5 boards that I've assembled is indeed due to thermal stress during reflow and not an inherent flaw in the circuit).

Should I give up trying to do this with a hot air rework station and buy a reflow oven?

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  • \$\begingroup\$ If you are not sure if you cooked them, stop using the hot air and solder your zeners more traditional way as it is only sot-23. But the hotair should not destroy your parts, and I think that the problem lays somewhere else - most likely in your design \$\endgroup\$
    – 0___________
    Jul 28, 2017 at 20:35
  • \$\begingroup\$ If there's only 20 parts and none of them are BGA, do it with an iron? Do you maybe have ESD problems during assembly? \$\endgroup\$
    – pjc50
    Jul 28, 2017 at 21:01
  • \$\begingroup\$ Along with what the others are saying... would definitely at least hand solder the parts that are causing you problems \$\endgroup\$
    – DigitalNinja
    Jul 28, 2017 at 21:02
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    \$\begingroup\$ How confident are you that it's not the layout and/or design? It might be time to post the schematic... unless you're doing something gravely wrong, hand soldering these components should not destroy them. \$\endgroup\$
    – DigitalNinja
    Jul 28, 2017 at 23:06
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    \$\begingroup\$ Pretty confident. The circuit works in tina-ti simulator. It works on a breadboard with the through-hole versions of the same devices. One of the five PCBs works flawlessly. I soldered the SMD zener to a breakout board to breadboard with it (built the test circuits from the datasheet). First attempt at that fried the zener and that was apparent in the test circuit. Second attempt yielded a zener on a breakout board that works. The zener feels like it is extra picky about getting heated up for whatever reason. \$\endgroup\$
    – macdonaldtomw
    Jul 29, 2017 at 1:46

1 Answer 1

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Using hand soldering worked for me. I'm guessing that I need to play around with my hot air soldering station a bit more in order to get a feel for what the actual temperature it outputs is compared to what the digital display shows. I'm guessing that the output temp is higher than what the meter says

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    \$\begingroup\$ A bit late but I would also comment that setting 280C on your station does not give you 280C on the board. I generally set my hot air station to 360-380C. Once the board heats up to the solder melting point, the solder will flow and your components should snap into position. \$\endgroup\$
    – Minho
    Sep 21, 2017 at 15:11

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