Learning objectives
- Describe how solder paste transfers from a stencil aperture onto a board pad
- Set and judge squeegee pressure, squeegee speed and separation behaviour
- Recognise a good print by deposit coverage, shape and registration
- Match common print defects to their causes and correct them at the printer
Lesson
Printing looks simple and decides more yield than any other single step in surface mount assembly. Industry experience treats paste volume as the single biggest lever on final quality, because most solder joint defects are born here as too much paste, too little paste, or paste in the wrong place. The equipment is a stencil printer, fully or semi automatic, a thin stainless steel stencil that is often nano coated to help paste release, and a squeegee, metal or rubber, that works the paste. The stencil carries one aperture for every pad, designed following the principles of IPC 7525, and the stencil thickness together with the aperture area ratio sets how much paste each deposit can hold.
The print cycle runs the same way every time. The board is clamped and aligned to the stencil using fiducial marks. The squeegee travels across the stencil and pushes a rolling bead of paste ahead of it. Pressure forces paste down into the apertures so each one fills completely against the pad. Then the board separates from the stencil, straight down and at a controlled speed and distance, and the paste deposits stay standing on the pads with clean, brick like shapes. Between prints, an underside wipe system cleans the stencil so paste does not build up, smear around the apertures and spoil the seal, called gasketing, between stencil and board.
A good operator runs the process, not just the machine. Paste is prepared properly, worked or kneaded as its maker directs, kept within its working life, and never mixed carelessly with returned paste. Print pressure and speed are set so apertures fill without the squeegee scooping paste back out. The operator watches the first prints closely, because a clogged aperture repeats its missing deposit on every board until someone cleans the stencil. James Onsomu teaches in his PCB quality book that applying solder paste is the preparation step that directly decides the quality of the solder joints formed later in reflow and wave soldering. A good print is easy to describe: every pad covered fully, deposits of even height and clean shape, no smearing between pads, and registration centred on the pads. Anything less is a defect being shipped to the placement machine.
Key numbers
- Paste volume: the primary yield lever of the whole surface mount process, set by stencil thickness and aperture design
- Print environment: temperature and humidity held controlled and steady during the shift
- Underside wiping: run on the programmed cycle and whenever print quality begins to drift
- Separation: straight downward movement at controlled speed and distance, so deposits release cleanly
Defects to spot
- Insufficient paste, leading to opens and weak joints: clogged apertures or low print pressure
- Excess paste and slump, leading to bridges and solder balls: apertures too large, worn stencil, or paste slumping warm
- Smeared deposits: poor gasketing between stencil and board
- Offset prints: fiducial alignment drift or board clamping faults
- Dog ears on deposits: separation speed too fast, pulling paste peaks upward
- Dry, crumbling paste: paste left open past its working life
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. Why is paste volume called the biggest yield lever in surface mount assembly?
A. Paste is the most expensive material on the line
B. Most solder joint defects begin as too much, too little, or misplaced paste at printing
C. It decides the colour of the finished board
D. It sets the speed of the placement machine
Show answer
Answer: B
2. One pad receives a missing deposit on every board. What is the most likely cause?
A. A clogged stencil aperture
B. A slow placement nozzle
C. A hot oven zone
D. A worn test probe
Show answer
Answer: A
3. What does gasketing mean at the printer?
A. The stencil is stored in a sealed bag
B. The squeegee is cleaned with solvent
C. The tight seal between stencil and board that stops paste smearing underneath
D. The board is clamped at its corners only
Show answer
Answer: C
4. Deposits show tall peaks, called dog ears, after printing. Which setting should be checked first?
A. The conveyor width
B. The separation speed and distance as the board leaves the stencil
C. The colour of the paste jar label
D. The height of the finished rack
Show answer
Answer: B
5. What is correct practice with paste that has been worked on the stencil?
A. Always mix it back into the fresh jar without checking
B. Use it first and check the date later
C. Leave it open overnight to rest
D. Follow the paste maker directions and never mix returned paste into fresh paste carelessly
Show answer
Answer: D