Epoxy Blob Excised Out Of Broken Multimeter, Replaced With A QFP

On the left, an image of a COB on the multimeter's PCB. On the right, a QFP IC soldered to the spot where a COB used to be, with pieces of magnet wire making connections from the QFP's pins to the PCB tracks.

The black blobs on cheap PCBs haunt those of us with a habit of taking things apart when they fail. There’s no part number to look up, no pinout to probe, and if magic smoke is released from the epoxy-buried silicon, the entire PCB is toast. That’s why it matters that [Throbscottle] shared his journey of repairing a vintage multimeter whose epoxy-covered single-chip-multimeter ICL7106 heart developed an internal reference fault. When a multimeter’s internal voltage reference goes, the meter naturally becomes useless. Cheaper multimeters, we bin, but this one arguably was worth reviving.

[Throbscottle] doesn’t just show what he accomplished, he also demonstrates exactly how he went through the process, in a way that we can learn to repeat it if ever needed. Instructions on removing the epoxy coating, isolating IC pins from shorting to newly uncovered tracks, matching pinouts between the COB (Chip On Board, the epoxy-covered silicon) and the QFP packages, carefully attaching wires to the board from the QFP’s legs, then checking the connections – he went out of his way to make the trick of this repair accessible to us. The Instructables UI doesn’t make it obvious, but there’s a large number of high-quality pictures for each step, too.

The multimeter measures once again and is back in [Throbscottle]’s arsenal. He’s got a prolific history of sharing his methods with hackers – as far back as 2011, we’ve covered his guide on reverse-engineering PCBs, a skillset that no doubt made this repair possible. This hack, in turn proves to us that, even when facing the void of an epoxy blob, we have a shot at repairing the thing. If you wonder why these black blobs plague all the cheap devices, here’s an intro.

We thank [electronoob] for sharing this with us!

16 thoughts on “Epoxy Blob Excised Out Of Broken Multimeter, Replaced With A QFP

  1. If it must be stuck down, to avoid dislodging it, I vote for using clear enamel, rather than apoxy, show off the work and make the chip details visible for any potential future hacker to see.

    1. I don’t know if it’s important in this case, but as a general rule sensitive analog semiconductors should not be allowed to be exposed to even low levels of light unless their purpose is to detect light. Light + semiconductor junction –> current.

      1. It is not important in this case. What was originally under the dark blob was most likely a raw silicon chip, which would need to be protected. What it was replaced with, however, was an integrated circuit. The raw silicon chip of an integrated circuit is first connected to the solderable pinouts and then the package is encased in typically black plastic epoxy or thermoset, leaving only the solderable pinouts exposed. Bottom line is the silicon is already protected, securing it with clear enamel or epoxy would be primarily to show off the soldering work and perhaps the part number.

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