ESD Failures After Assembly in a Dry Workshop
In a dry-season workshop, relative humidity is 30%. Twelve students assemble 20 boards containing a 74HC595 shift register and a CD4017 counter. There is an ESD wrist strap at the bench, but its clip is broken, so students work without it. They wear synthetic sweaters and rubber-soled shoes, and ICs are kept in clear plastic bags. The floor is vinyl. A soldering iron at the bench has a three-pin plug, but the socket's earth pin is loose. Three boards fail after two weeks: one 74HC595 output stuck high, one CD4017 resets randomly. The failures cannot be reproduced by tapping or heating. The supervisor wants to continue because the parts are cheap.
1. Identify the ESD risks in this setup and explain how they could cause the later failures.
Read the worked answer
The main risks are no working wrist strap, synthetic sweaters, rubber-soled shoes, a vinyl floor, plastic bags, and a loose earth. Walking on vinyl with synthetic clothing can charge a person to several kilovolts. When that person touches an IC pin, the charge discharges through the gate oxide.
The 74HC595 and CD4017 are CMOS devices and are sensitive to ESD. Damage can be latent: a weakened oxide or bond may pass the first test and fail days later. That matches the stuck output and random resets, which cannot be reproduced by tapping or heating. Cheap parts do not remove the cost of rework, returns, and unreliable products.
2. Describe the ESD controls you would put in place, including what to do about the broken strap and the loose earth.
Read the worked answer
Stop assembly until the controls are fixed. Replace the wrist strap; a proper strap has a 1 MΩ series resistor. Connect it to a verified earth point, not to a loose pin. Use a dissipative bench mat connected to the same earth through 1 MΩ. Keep ICs in anti-static tubes or foam, not clear plastic bags.
Have a qualified electrician repair the socket earth and check the soldering iron for tip leakage before use. Do not rely on the wrist strap until the earth is verified. Use cotton clothing, avoid synthetic sweaters, and raise humidity or use an ionizer if the dry condition continues.
3. What should be done with the failed boards and the rest of the batch? Should production continue?
Read the worked answer
Quarantine all 20 boards. The three failures are suspect; replace the CMOS ICs and retest. Because ESD damage can be latent, functionally test or burn-in the other boards and record the results.
Do not ship until the ESD controls are in place and the earth is verified. The supervisor's argument that the parts are cheap ignores the cost of returns, reputation, and safety. Production should continue only after the workstation checklist includes a wrist-strap test, anti-static storage, and a check that the soldering iron and bench are properly earthed.