What is BGA assembly
A ball-grid array (BGA) is an IC package with solder balls arranged in a grid on the underside — no leads on the edges. The balls connect to matching pads on the PCB during reflow. Pitch ranges from 1.27 mm (older PBGAs) down to 0.4 mm (modern µBGAs), with ball counts from 36 to 2000+.
The challenge: every solder joint is under the package. You cannot see them. You cannot probe them. AOI is useless. X-ray and electrical test are the only post-reflow inspection methods.
At Energetika-VDS we assemble BGAs from 6 × 6 mm 0.5 mm pitch up to 45 × 45 mm 1.0 mm pitch on the DDM Novastar LS60 head with GF-120HT 8-zone reflow. X-ray inspection is sourced through partners — every board on Class 3 orders, sample-based on Class 2.
BGA assembly step-by-step
1. Paste print
Stencil aperture for BGA pads: typically 1:1 to the pad diameter, sometimes -5% for fine-pitch (≤0.5 mm) to reduce bridging. Stencil thickness:
| BGA pitch | Stencil thickness |
|---|---|
| 1.0-1.27 mm | 125-150 µm |
| 0.65-0.8 mm | 100-125 µm |
| 0.4-0.5 mm | 75-100 µm with electroform |
Paste volume target: 80-110% of nominal, measured by SPI if available. Below 70% = open joints. Above 130% = bridging on fine pitch.
2. Placement
The LS60 picks the BGA, vision-centres on the ball pattern, and places at ±30 µm @ 3σ. The BGA does not need to be perfect — self-alignment during reflow pulls the package onto the pads if placement is within 50% of pad diameter. We aim for under 25%.
Placement speed: 0.5-1.2 s per BGA depending on ball count and centring complexity.
3. Reflow
SAC305 BGAs follow the standard reflow profile from our SMT process article:
| Zone | Temperature | Duration |
|---|---|---|
| Preheat | 25 to 150°C | 60-90 s |
| Soak | 150-200°C | 60-120 s |
| Reflow peak | 235-245°C | 30-60 s above 217°C |
| Cooldown | 245 to 50°C | 60-120 s |
Time-above-liquidus (TAL): 45-90 s is the sweet spot. Below 45 s = cold joints in the centre of large BGAs. Above 90 s = excessive intermetallic growth and increased voiding.
4. X-ray inspection
After reflow, BGAs go to 2D X-ray for visual inspection of:
- Ball presence (no missing balls)
- Voiding per ball
- Bridging between balls
- Head-in-pillow (ball did not wet to pad)
- Open joints (insufficient solder)
2D X-ray catches 90%+ of BGA defects. 3D / CT X-ray catches the rest (head-in-pillow on inner rows of large packages). We source 2D X-ray as standard; 3D on request via partner labs.
5. Functional test or boundary scan
If the design includes JTAG boundary scan, every net between BGA balls and adjacent components is tested electrically. This catches opens and shorts X-ray cannot see (cold joints that look fine geometrically). FCT itself is sourced through partners at Energetika-VDS.
Voiding — the real metric
Voids are bubbles of trapped flux gas inside a reflowed solder ball. Some voiding is unavoidable. The question is how much.
| Standard / Class | Void limit per ball |
|---|---|
| IPC-A-610 Class 2 | ≤25% projected area |
| IPC-A-610 Class 3 (default) | ≤25% area |
| Class 3 with customer spec | Often ≤9-15% area |
| Power BGAs (thermal pad) | ≤30% area on signal balls, ≤50% area on thermal |
| Automotive / medical (custom) | Often ≤9% area |
Top 4 voiding causes
Tented vias under BGA. Vias filled only with solder mask leak flux gas into the joint during reflow. Solution: fill and cap (IPC-4761 Type VII) or relocate vias outside the BGA shadow. Caught in our DFM checklist.
Wrong stencil thickness. Too thick = excess paste = excess flux = more voids. Too thin = insufficient solder = head-in-pillow. Match stencil to pitch (table above).
Oxidised pads or balls. Pads stored over 12 months without HASL or with poor ENIG plating wet poorly, leaving voids at the pad interface. Always use fresh boards from a known fab — NCAB and Eurocircuits both run pad-oxidation tests.
Reflow profile with insufficient TAL. Below 45 s above liquidus, flux gases cannot escape. Above 90 s, intermetallics grow into voids. Target 60-75 s for most SAC305 BGAs.
BGA rework
Rework station: hot-air or IR nozzle matched to BGA dimensions. Cycle:
- Pre-bake board 4-8 h at 110°C to drive out moisture (MSL Level 3 parts)
- Heat BGA to 245°C for 30-60 s, lift with vacuum nozzle
- Wick remaining solder, clean pads
- Re-ball the BGA (or use a new part)
- Re-paste pads (mini-stencil or dispensed)
- Place, reflow with local profile
- X-ray verify
Time per BGA rework: 30-60 minutes. Cost: €40-150 per joint depending on size and access. Get it right first time.
BGA vs QFN — when to choose what
| Factor | BGA | QFN |
|---|---|---|
| Pin count | 36 to 2000+ | 8 to 100 |
| Inspection | X-ray required | AOI + side fillet |
| Rework cost | €40-150 | €5-20 |
| Cost of part | Higher | Lower |
| Thermal performance | Good (thermal balls) | Excellent (thermal pad) |
| RF performance | Better (shorter loops) | Good |
For high pin counts (>100) and high-speed digital, BGA is mandatory. For 20-80 pin power, RF, and analog ICs, QFN is cheaper and easier to inspect.
Submit your design — DFM review flags BGA-specific issues (tented vias, escape routes, stencil aperture) before tooling. Try the quote estimator first for a price band.
Frequently asked questions
What is BGA? Ball-grid array — an IC package with solder balls arranged in a grid on the underside. Connects to matching PCB pads during reflow. Pitch ranges from 0.4 mm to 1.27 mm; ball counts from 36 to 2000+.
BGA vs QFN — which is better? Neither. BGA for high pin counts (>100), high-speed digital, and RF. QFN for lower pin counts (8-100) with excellent thermal performance through a centre pad. BGA rework is 5-10× more expensive than QFN, so choose QFN where it fits.
How do you inspect a BGA? X-ray (2D for standard, 3D / CT for head-in-pillow detection on large packages). AOI cannot see under the package. Boundary scan (JTAG) catches electrical opens and shorts the X-ray cannot see geometrically.
What is BGA voiding? Bubbles of trapped flux gas inside a reflowed solder ball. Some voiding is unavoidable. IPC-A-610 Class 2 allows up to 25% projected area per ball; Class 3 with customer spec often demands under 9-15%. Causes: tented vias, oxidised pads, wrong stencil thickness, insufficient time above liquidus.