Learning objectives
- Inspect incoming boards and components against the bill of materials and the approved vendor list
- Set up and work correctly inside an ESD protected area
- Read a humidity indicator card and manage component floor life by moisture sensitivity level
- Store and handle solder paste correctly, including refrigeration and warm up before use
Lesson
### Lesson 1: Incoming inspection and traceability
Quality starts at the receiving door, before anything reaches the line. Incoming inspection compares bare boards and components against the bill of materials and the approved vendor list, because a correct looking reel from an unapproved source can carry counterfeit or mixed parts. Date codes and lot codes are recorded for every material, and that record is the backbone of traceability. If a field failure appears months later, those codes are how a manufacturer proves which lots went into which products. Bare boards are stored flat and dry, following the principles of IPC 1601, so they arrive at the printer clean, flat and free of absorbed moisture.
### Lesson 2: ESD control
Electrostatic discharge can wound a component without destroying it, and the wounded part passes test and fails early in the customer’s hands. That is why assembly areas are built as ESD protected areas under programmes aligned to ANSI ESD S20.20 and IEC 61340. Operators wear tested wrist straps or footwear, work on dissipative mats, use ionisers where insulators cannot be removed, and move parts only in ESD safe packaging. The rule is simple to state and strict in practice: no unprotected handling, ever, because the discharge that damages a device is usually far below what a person can feel.
### Lesson 3: Moisture control and paste handling
Moisture sensitive devices absorb water from the air while they wait. In reflow, that water turns to steam inside the package and can crack it, a failure called popcorning, or lift its internal bonds and cause delamination. Manufacturers classify parts by moisture sensitivity level under IPC JEDEC J STD 020, and handling, packing and baking follow IPC JEDEC J STD 033. Parts live in sealed moisture barrier bags with desiccant and a humidity indicator card. The card is read the moment a bag opens, the floor life clock starts, and the exposure is logged. When floor life expires, the parts are baked dry before use rather than gambled on. Solder paste gets the same respect. It is kept refrigerated, brought to room temperature while still sealed so condensation stays out of it, and scrapped when its shelf life ends. James Onsomu teaches in his PCB quality book that quality control belongs at the earliest stage of the line, precisely because a problem caught here costs almost nothing, while the same problem found after reflow costs rework, delay and trust.
Key numbers
- Floor life at 30 degrees Celsius and 60 percent relative humidity or below: MSL 3 allows 168 hours, MSL 4 allows 72 hours, MSL 5 allows 48 hours
- Humidity indicator card: read and recorded every time a moisture barrier bag is opened
- Dry cabinet storage: held at low humidity, below 10 percent relative humidity and often near 5 percent
- Solder paste: refrigerated in storage, warmed to room temperature while still sealed, never used past its shelf life
Defects to spot
- Popcorning and delamination in reflow: floor life clock not logged, so moisture loaded parts were soldered
- Counterfeit or mixed reels on the line: approved vendor and label checks skipped at receiving
- Expired paste used: shelf life dates not checked before the jar was opened
- Torn or dry moisture packs ignored: bag damage seen but never reported
- Latent ESD damage: unprotected handling outside the ESD protected area
Stage check, self test
Answer each question, then open the answer to check yourself. Passing standard: 80 percent, at least 4 of 5 correct.
1. A part is classified MSL 4. What is its floor life at 30 degrees Celsius and 60 percent relative humidity?
A. 168 hours
B. 48 hours
C. 72 hours
D. Unlimited, if the bag looked dry
Show answer
Answer: C
2. What should happen the moment a moisture barrier bag is opened?
A. The humidity indicator card is read and the floor life clock starts
B. The parts are placed on any open shelf
C. The desiccant is thrown away immediately
D. The parts are washed before use
Show answer
Answer: A
3. Why is ESD damage so dangerous even when the board passes its tests?
A. It always shows as a visible spark
B. It only affects the packing stage
C. It makes labels fall off
D. A wounded component can pass test and fail early in service
Show answer
Answer: D
4. How should refrigerated solder paste be prepared for use?
A. Opened cold and used at once
B. Warmed in an oven with the lid off
C. Brought to room temperature while still sealed, so condensation stays out
D. Mixed with fresh paste to extend its life
Show answer
Answer: C
5. What is the correct response when a moisture barrier bag arrives torn?
A. Use the parts quickly before anyone notices
B. Report it and treat the contents as moisture exposed until assessed
C. Tape the bag and return it to stock without a note
D. Ignore it, bags tear often
Show answer
Answer: B
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