Laser solder ball mounting is a localized reflow process: a pre-formed solder ball is placed on a fluxed pad and heated by a focused laser spot until it collapses, wets and forms a joint — without running the whole board…
Laser solder ball mounting is a localized reflow process: a pre-formed solder ball is placed on a fluxed pad and heated by a focused laser spot until it collapses, wets and forms a joint — without running the whole board through a reflow oven. It is chosen when paste printing cannot deliver enough solder volume, when the assembly cannot survive an oven cycle, or when a single joint on an already-populated board has to be reworked. Typical applications are BGA/CSP reballing, package and die-level interconnects, camera and optical modules, sensor packages, and individual connector or shield pads on automotive control boards.
What Does a Laser Solder Ball Mounting Process Look Like Step by Step?
A production cell usually runs six steps, at roughly 0.3–1.5 s per joint plus indexing time:
- Flux deposition — pin transfer, jetting or time-pressure dispensing, typically a 60–200 µm dot centred on the pad. Flux volume is the single most under-estimated variable; too much causes spatter and voids, too little causes non-wetting.
- Ball feeding and singulation — hopper or vibratory feed into a nozzle plate or die, vacuum pickup, and a presence check by vacuum sensor or vision before the head moves.
- CCD alignment — fiducial-based correction of both board and ball position; ±20 to ±50 µm placement accuracy is a realistic production figure on a well-calibrated machine.
- Ball placement — the nozzle descends to 30–100 µm above the pad, then releases with vacuum break plus a light positive puff. The flux tack, not the nozzle, must hold the ball in place.
- Laser reflow — the spot is centred on the ball, or offset 20–30% of the ball diameter toward the pad edge to promote wetting, with pyrometer-based closed-loop power control.
- Inspection — AOI for position, collapse and wetting angle; X-ray for voids on sampled or highly basis; shear test on first-article samples.
For reballing, add ball removal (hot gas or laser), pad cleaning to bare copper or finish, and a flatness check on the package substrate before the first new ball goes down.
How Should Laser Power, Spot Size and Timing Be Set?
The window is set by ball mass, alloy, pad finish and absorptivity — not by an oven profile. Absorptivity of a shiny solder ball at 915–980 nm is only around 10–20% and rises as the surface heats and oxidises, which is why open-loop power control produces a 20–30 °C spread across nominally identical joints. Closed-loop control sampling at 1–5 ms, with the pyrometer aimed at the ball rather than the surrounding mask, removes most of that spread.
| Parameter | Typical range | Practical note |
|---|---|---|
| Ball diameter | 0.15–1.00 mm (0.25/0.30/0.40/0.50 most common) | Below 0.15 mm, ball handling and flux tack dominate the difficulty, not the laser |
| Pad diameter | 0.7–0.9 × ball diameter | Copper-defined pads wet more evenly; mask-defined pads need slightly more energy |
| Laser spot diameter | 1.2–2.0 × ball diameter | Spot smaller than the ball concentrates energy and can crater the pad |
| Power at work surface | 5–60 W | Scales with ball mass; verify at the workpiece, not at the source |
| Irradiation time | 20–300 ms | Longer, lower-power pulses reduce spatter on large balls |
| Heating rate | 100–400 °C/s | Above ~500 °C/s, flux spitting and micro-ball spatter increase sharply |
| Peak temperature | Liquidus + 20–40 °C | SAC305 ≈ 240–250 °C; Sn63Pb37 ≈ 200–215 °C; SnBi ≈ 155–170 °C; AuSn20 ≈ 300–320 °C |
| Time above liquidus | 0.3–2 s | Long enough to collapse and wet, short enough to limit intermetallic growth |
| Nitrogen shielding | O₂ < 1000 ppm (fluxless: < 100 ppm) | Measure at the nozzle outlet, not at the regulator |
Which Ball, Pad and Flux Choices Decide Yield?
- Pad finish: ENIG and immersion tin wet predictably; OSP needs more flux or more energy and is far less forgiving of a second heating pass.
- Ball alloy: match the peak temperature to the alloy, and confirm that the ball alloy is compatible with the existing joint on a mixed assembly — a low-melting SnBi ball next to a SAC305 joint will re-melt in later processing.
- Flux: no-clean fluxes with low outgassing reduce voids but have shorter tack life; water-soluble fluxes give better wetting on oxidised pads but require cleaning that may be impossible under a package.
- Ball-to-pad volume: the ball should contain roughly the solder volume the joint needs after collapse. Oversized balls on tight pitch are the usual cause of bridging.
- Board support: a vacuum or pin support directly under the pad prevents the substrate flexing away from the ball as flux volatiles escape.
Which Failure Symptoms Point to Which Parameter?
| Symptom | Likely cause | Corrective action |
|---|---|---|
| Ball not collapsed / cold joint (dull, grainy) | Peak temperature short of target, spot misalignment, thick oxide, too little flux | Raise peak 10 °C, re-verify pyrometer against a thermocouple coupon, check spot centring, increase flux dot |
| Solder splash or micro-ball spatter | Heating rate too high, moisture in board or flux, flux over-volume | Reduce ramp, pre-bake boards at 100–125 °C, cut flux volume, check N₂ flow at nozzle |
| Voids above acceptance limit | Flux outgassing, short time above liquidus, contaminated pad, via-in-pad | Extend time above liquidus, switch to lower-void flux, clean pads, consider vacuum assist |
| Ball shifted off pad | Placement offset, insufficient flux tack, late vacuum release, board vibration | Re-teach placement, increase flux tack, advance vacuum break, stiffen fixturing |
| Bridging on fine pitch | Excess solder volume, spot heating neighbouring ball, flux spread | Step down ball size, reduce spot diameter, reduce flux dot |
| Pad cratering or mask damage | Over-power or long dwell with a small spot | Increase spot size, reduce power, shorten pulse, re-check spot-to-ball ratio |
| Brittle joint / thick intermetallic | Excessive peak or time above liquidus | Cut time above liquidus to under 2 s, check cross-section IMC at 1–3 µm for Cu₆Sn₅ |
What Acceptance Checks Should Run at Setup and In-Process?
- Temperature verification: cross-check the pyrometer reading with a fine thermocouple on a sacrificial ball, or use a melting-point standard; agreement within ±10 °C is the target.
- First article: 10 joints at the extremes of the panel — AOI for position and wetting angle, X-ray for voiding, collapse height within ±10% of nominal.
- Shear test: per JESD22-B117, recording both force and failure mode; a ductile ball-shear failure is the expected result, brittle fracture indicates an IMC or contamination problem.
- Cross-section: periodic sampling to confirm IMC thickness and check for fillet voids under the package edge.
- Daily: nozzle cleaning, ball-feed verification, flux pot life check, and a review of AOI false calls — a rising false-call rate usually means flux build-up on the nozzle.
FAQ
Can laser solder ball mounting be used to reball BGA and CSP packages?
Yes. It is one of the main uses, because the heat is confined to the joint being formed and adjacent joints on the same package do not reflow. Ball sizes of 0.25–0.76 mm are routine; the limiting factor is usually pad cleaning quality on the removed site, not the laser.
What is the smallest solder ball the process can handle?
Around 0.15 mm in production, with a nozzle bore of 0.10–0.12 mm. Below that, ball singulation, vacuum pickup reliability and flux tack become the practical limits rather than the optical or thermal ones.
Is flux always required?
No. With nitrogen below 100 ppm O₂ and oxide-free balls, fluxless bonding is possible and is used in hermetic and optical packages, sometimes with formic acid assist. On standard FR-4 assemblies, a small flux dot is still the most reliable way to get consistent wetting.
How do I verify joint temperature without attaching a thermocouple to every ball?
Rely on closed-loop pyrometer control for production, and calibrate it at line start against a thermocouple or melting-point standard on a coupon. Collapse time measured by the vision system is a useful secondary indicator — a sudden increase in collapse time at constant power usually means the optics or the nozzle shield needs cleaning.
What cycle time should I expect?
0.3–1.5 s of laser and motion time per ball, depending on ball size and alloy. Throughput is normally gained by adding heads or parallel indexing rather than by shortening the pulse below the wetting window.
If you share your ball size, alloy and pad finish, we can help you define a starting process window and the acceptance checks to run on your first article.