Depositing paste on a BGA footprint with a mini stencil
Re-balling with solder balls is the textbook method and the slowest one. A 0.12 mm stainless mini stencil and a squeegee gets the right paste volume onto a 0.35 mm pad field in under a minute, and a properly printed deposit reflows with lower voiding than hand-placed balls.
Procedure
- 1
Clean the pad field with isopropyl alcohol and let it dry completely.
- 2
Align the 0.12 mm stencil under 20x on two corner references and tape two opposite edges down.
- 3
Apply Type 4 SAC305 paste and pull one firm pass with the squeegee at 60 degrees.
- 4
Lift the stencil straight up at about 2 mm per second, keeping it level.
- 5
Inspect at 40x with side light for missing, bridged or smeared deposits, then clean the stencil with alcohol before the next print.
Workshop notes
Aperture design and paste volume
The stencil aperture should be 0.30 mm square for a 0.35 mm pad, giving an area ratio of about 0.62 with a 0.12 mm stencil - just above the 0.6 threshold where paste releases cleanly. Apertures equal to the pad (0.35 mm) look better on paper but release inconsistently and leave paste smeared across the mask instead of on the pad. Ask the stencil supplier for a 4 mil foil with 0.30 mm apertures and a 0.08 mm web.
Printing on a hand-aligned stencil
Clean the pad field with alcohol, then align the stencil under 20x using two corners as references and tape it down on two opposite edges. Hold the squeegee at 60 degrees, one firm pass with a Type 4 paste at 20 to 25 degrees C, and lift the stencil straight up at 2 mm per second. Lifting at an angle smears the deposit; lifting fast pulls paste out of the apertures as a string.
Inspection before placing the package
Look at the deposit under 40x with side light. Each aperture should have a flat, slightly rounded top with no bridging to its neighbour and no missing deposits. A deposit that is domed and taller than the stencil thickness means the stencil was lifted too slowly; a deposit with a crater in the middle means the paste was too cold or has passed its working life. Paste on the mask between pads is the most common defect and can be picked off with a fine probe.
Practice drill
Print five footprints in a row with the same stencil, inspecting each at 40x before reflow, and count missing, bridged and smeared apertures. A good session has fewer than three defects across five prints; more than that and the alignment method or the squeegee angle needs changing.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Paste smeared across the mask between pads | Stencil lifted at an angle or too fast, or the squeegee pass ran twice over the same area | Clean and reprint: one pass only, lift straight up at 2 mm per second, and remove stray paste with a fine probe before placing the package |
| Deposits come out domed and too tall | Stencil released too slowly, so paste crept back up into the aperture as it lifted | Increase the lift speed to 2 mm per second, hold the stencil level, and reduce the squeegee pressure slightly |
| Apertures empty in one corner of the field | Stencil not flush with the board at that corner, so paste pushed under the foil instead of through it | Re-tape that edge, check for a solder bump or stray ball under the stencil, and add a shim if the board is warped |
Supplies
- Type 4 SAC305 solder paste in a 10 cc syringe
- 0.12 mm stainless mini stencil with 0.30 mm apertures
- Isopropyl alcohol and lint-free wipes
- Fine probe for removing stray paste
Tools
- Metal squeegee with a 60 degree edge
- 40x stereo microscope with side illumination
- Stencil frame or tape for holding the stencil
- Station at 300 degrees C for stencil-edge cleanup
Safety
Solder paste is a fine metal powder in a solvent carrier. Never expose it to moisture, never store it open next to food, and wash hands thoroughly after handling - the lead-free label refers to the alloy, not to the paste vehicle.
Neighbouring cards
- Corner-tack placement before BGA reflow 340 °C / 1.0 s
- Wicking and dressing a BGA pad field 360 °C / 1.5 s
- Hot-air reflow profile for a 0.8 mm BGA 380 °C / 3.0 s