Wicking and dressing a BGA pad field
After removing a BGA, the pad field must be flat to within about 10 microns or the replacement will not sit evenly. Wicking is easy; dressing without tearing pads is the part that takes practice, because a 0.35 mm pad gives you almost no margin.
Procedure
- 1
Cut 2.0 mm braid into 8 mm segments and lay one across the field.
- 2
Add RMA flux and slide the braid across the field in one direction with a 2.0 mm knife tip at 360 degrees C.
- 3
Change the braid segment every two strokes; never use a filled segment on a 0.35 mm pad.
- 4
Rotate the work 90 degrees and run a second wicking pass, then dress each row with a clean braid at the weight of the iron only.
- 5
Inspect at 40x with grazing light for bright or dark patches, then tin thinly if the field will not be populated immediately.
Workshop notes
Wicking the field with a wide braid
Use 2.0 mm braid cut into 8 mm segments and a 2.0 mm knife tip at 360 degrees C. Slide the braid across the field in one direction with light pressure and change the segment every two strokes. A 0.35 mm pad releases its solder in under a second; the temptation to hold longer comes from watching the field rather than the clock. Work in one direction only until the whole field is dry, then rotate 90 degrees and do a second pass.
Dressing flat without thinning the pads
Dressing removes the last few microns of alloy and any intermetallic that formed. Lay a clean 2.0 mm braid over the field, add RMA flux, and stroke with the knife edge at a shallow angle, one pass per row. Pressure should be the weight of the iron alone - pressing down burnishes the copper and makes the pad surface shiny but no longer wettable, which shows up later as a perfectly flat pad that refuses to take a new ball.
Post-dress inspection and re-tinning
Inspect at 40x with grazing light: the pads should be uniformly copper-coloured with no bright silver patches (leftover alloy) and no dark patches (oxidised or burnished copper). If the field will not be re-populated immediately, tin it with a very thin film of Sn63Pb37 and store the board in a bag with desiccant - bare copper pads oxidise within days in humid air.
Practice drill
Take a scrap BGA footprint and wick and dress it five times, inspecting at 40x after each pass and counting pads with visible damage. A good result is zero damaged pads on four out of five passes; the damage you should look for first is thinning at the pad edge where the braid caught it.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Pad surface shiny but will not accept a new ball | Burnished copper from pressing harder than the weight of the iron during dressing | Reduce to iron weight only, add fresh RMA flux, and re-tin with a very thin film of Sn63Pb37 at 330 degrees C |
| Pads thinned or torn at the edges | Filled braid segment reused for more than two strokes, or braid dragged sideways | Change segments every two strokes and always stroke in one direction only, lifting the braid before repositioning it |
| Field oxidises before the new package arrives | Bare copper left exposed in humid air for more than a day | Tin thinly with Sn63Pb37 immediately after dressing and store the board bagged with desiccant |
Supplies
- 2.0 mm RMA-fluxed desoldering braid
- RMA liquid flux
- Sn63Pb37 wire for the protective re-tin
- Isopropyl alcohol and lint-free swabs
- Desiccant bags and antistatic bags
Tools
- Station at 360 degrees C
- 2.0 mm knife tip
- 40x stereo microscope with side illumination
- Flush cutters for braid segments
Safety
Repeated reflow and wicking on the same footprint accumulates flux residue that is both a skin sensitiser and a slipping hazard on a smooth bench surface. Clean the work area between boards, and wash hands before eating.
Neighbouring cards
- Corner-tack placement before BGA reflow 340 °C / 1.0 s
- Hot-air reflow profile for a 0.8 mm BGA 380 °C / 3.0 s
- Depositing paste on a BGA footprint with a mini stencil 300 °C / 1.0 s