SMTech SMT Stencil Printer Display Issues
Problem
A customer reported problems with the LCD on an automated stencil printer, where the display would take around 30 minutes to start showing correct information. The unit is from an SMTech Ltd automatic SMT solder paste stencil printer. We found a NiMH battery soldered on LCD PCB had leaked and nearby traces had corroded. It also appeared like someone had previously attempted repairs and lifted some tracks and pads, as well as abrading the PCB quite badly.
Diagnosis
We repaired the traces with enamelled copper wire and flowed solder on the corroded traces. Once fixed, we covered the repaired area with UV curable solder mask. There was also an invisible short circuit somewhere on the LCD contrast control which was fixed by removing components including a capacitor which had clearly been replaced before, which cleared the short. Evidently there was a small solder bridge somewhere.
At this stage the LCD was once again visible and contrast was controllable. The PCB was still sensitive to any flex, resulting in comedy garbage on the display. The backlight draws 2 amps at 5 volts, which is another clear indication of problems in the LCD module.
Solution
A replacement LCD module was obtained and fitted. With the replacement installed there were still intermittent issues which were traced to the MCU socket making poor contact. Contact cleaner and a tiny shim was used to improve the connection of this socket. This PCB is now working as expected.
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