Corner-tack placement before BGA reflow
A BGA placed on paste or flux will drift during the ramp unless it is mechanically held. Tacking the four corner balls with a fine iron before the hot-air pass is the cheapest way to hold alignment, and it does not disturb the ball field if you approach at a shallow angle.
Procedure
- 1
Align the package under 40x with grazing light from two sides and confirm all four corners are within 25 microns.
- 2
Set the bottom pre-heater to 120 degrees C so the board is warm before the iron touches it.
- 3
Bring a 0.4 mm conical tip in at 20 degrees to the surface and weld corner 1 for one second at 340 degrees C.
- 4
Tack corner 3 next, then corners 2 and 4, always working diagonally.
- 5
Re-check alignment at 40x before the hot-air pass; if any corner moved, reflow that tack and nudge with a fine probe.
Workshop notes
Alignment first, at 40x, with grazing light
Align the package so the corner balls sit within 25 microns of their pad centres. Grazing side light from two directions shows ball-to-pad offset far better than top light, because the shadow line moves with the offset. Check all four corners; a package aligned on three corners and 80 microns out on the fourth will reflow fine but pull the fourth corner ball into a stretched shape that fails under thermal cycling.
Tacking the corner without disturbing the field
Bring a 0.4 mm conical tip in at about 20 degrees to the board surface, so the tip contacts the corner ball and its pad from the side rather than pressing down through the package. One second at 340 degrees C is enough to weld the corner. Work diagonally - corner 1, then corner 3, then 2 and 4 - because tacking adjacent corners lets the package rotate slightly around the first joint.
When not to tack
If the board is going straight into a reflow oven with a full convection profile, skip the tack entirely: the tack joints are made with an iron that has no bottom heat, so they sit on a large thermal gradient and the surrounding balls see an uneven start. Tacking is for hot-air hand rework on a pre-heater, where the board is already at 120 degrees C and the gradient is small.
Practice drill
Place and tack ten BGAs on scrap boards, then measure the final alignment after a mock reflow using a top-down photo at 40x and image analysis, or simply a visual grid overlay. Track how many stay within 50 microns. Tacking should get you to 9 out of 10.
Failure modes
| Symptom | Cause | Treatment |
|---|---|---|
| Corner ball flattened or deformed by the tack | Tip approached from above with downward pressure instead of at a shallow side angle | Wick the deformed ball, re-ball the corner with 0.3 mm wire, and re-tack at 20 degrees with no downward force |
| Package rotated slightly around the first tack | Adjacent corners tacked in sequence rather than diagonally | Work 1-3-2-4, and re-check rotation at 40x after the second tack before proceeding |
| Balls around the tack show cold joints after reflow | Tack made without bottom heat, creating a local temperature gradient the profile cannot flatten | Use a 120 degrees C bottom pre-heater for the tack, or skip tacking entirely and go straight to a full convection profile |
Supplies
- ROL1 gel flux applied under the package before placement
- 0.3 mm SAC305 wire if the corner needs make-up solder
- Isopropyl alcohol and swabs
- Fine probe for nudging
Tools
- Station at 340 degrees C
- 0.4 mm conical tip
- 40x stereo microscope with dual side illumination
- Hot-air station with 120 degrees C bottom pre-heater
Safety
Tacking a BGA means bringing a fine iron under the package edge where you cannot see the pad. Keep the tip angle shallow so a slip goes sideways across the board rather than down into your fingers, and hold the board with a fixture, not by hand.
Neighbouring cards
- Wicking and dressing a BGA pad field 360 °C / 1.5 s
- Depositing paste on a BGA footprint with a mini stencil 300 °C / 1.0 s
- Hot-air reflow profile for a 0.8 mm BGA 380 °C / 3.0 s