BGA Assembly: A Complete Guide

BGA (Ball Grid Array) assembly is where precision manufacturing meets high-stakes electronics. Those tiny solder balls hiding beneath your processor need to land perfectly on hundreds of pads, melt at exactly the right temperature, and form reliable joints you can't even see. No pressure, right?

What Makes BGA Assembly Different?

Unlike through-hole components or even standard SMD packages, BGA assembly comes with unique challenges:

Hidden joints: You can't visually inspect solder connections after assembly. They're completely covered by the package.

No margin for error: With ball pitches as fine as 0.4mm, there's no room for misalignment. A few thousandths of an inch off, and you have opens or shorts.

Thermal sensitivity: Large BGA packages with hundreds of balls require careful thermal management during reflow.

Expensive failures: BGA components often cost $50-500+, making rework costly and time-consuming.

The BGA Assembly Process

Step 1: Stencil Design and Solder Paste

Solder paste application is critical. Too much paste causes bridges between balls; too little results in open connections.

Stencil specifications: Typically 100-150µm thick, laser-cut for precision. Apertures match BGA pad patterns with proper area ratios.

Paste selection: Type 3 or Type 4 solder paste for standard BGAs; Type 5 for ultra-fine pitch. Lead-free SAC305 (96.5% Sn, 3% Ag, 0.5% Cu) is common.

Printing parameters: Squeegee speed, pressure, and separation speed all affect deposit quality. Print and inspect before placement.

Step 2: Component Placement

Modern pick-and-place machines handle BGAs with vision-assisted placement:

Fiducial alignment: The machine uses fiducial marks on the PCB and package to calculate precise placement coordinates.

Placement accuracy: ±25-50µm is typical for fine-pitch BGAs. Better machines achieve ±10µm.

Self-centering: Surface tension during reflow helps correct minor misalignment—but only minor. Grossly misplaced BGAs won't self-correct.

Package orientation: A dot, notch, or marking identifies pin 1. Get this wrong, and you'll need a rework station.

Step 3: Reflow Soldering

The reflow profile is where science meets art. Each zone serves a purpose:

Preheat (25°C to 150-180°C at 1-2°C/sec): Gradual heating prevents thermal shock. Too fast, and you risk component cracking or solder splattering.

Soak (150-180°C for 60-90 seconds): Temperature equalization across the board. All balls need to reach liquidus simultaneously.

Reflow (peak 235-250°C for 30-60 seconds above liquidus): Solder melts, flows, and forms joints. Too hot or too long damages components and burns flux.

Cooling (2-4°C/sec): Controlled cooling prevents thermal stress cracks. Too fast causes joint embrittlement.

Nitrogen atmosphere: Reduces oxidation, improves wetting, and creates shinier joints. Not mandatory but beneficial.

Common BGA Assembly Defects

Solder Bridges

Adjacent balls connect, causing electrical shorts. Causes include:

• Excess solder paste
• Package misalignment
• Collapsed balls from excessive reflow temperature

Open Connections

No electrical connection between ball and pad. Causes:

• Insufficient solder paste
• Poor pad wetting from oxidation
• PCB or package warpage
• Head-in-pillow defect

Head-in-Pillow (HiP)

The ball melts but doesn't merge with the paste. Creates weak or intermittent connections. Common causes:

• Oxidized ball or paste surfaces
• Warpage preventing contact during reflow
• Reflow profile issues

Voids

Trapped gas bubbles within solder joints. Some voiding (under 25%) is acceptable, but large voids weaken joints and reduce thermal transfer. Causes:

• Outgassing from paste or flux
• Moisture in components or PCB
• Improper reflow profile

Pad Cratering

PCB laminate fractures under the pad, typically at the resin-glass interface. More common with lead-free solder due to higher reflow temperatures. Causes include:

• Excessive mechanical stress during placement or testing
• Thermal shock during rework
• Weak laminate material

BGA Inspection Methods

You can't see BGA joints, so specialized inspection is essential:

X-Ray Inspection

The gold standard for BGA inspection. X-rays penetrate the package and reveal:

• Ball presence and position
• Bridging between balls
• Voids within joints
• Open connections

2D X-ray: Good for basic inspection. Shows ball array from above.

3D X-ray (CT): Detailed tomographic imaging. Essential for complex assemblies with multiple BGA layers.

Automated Optical Inspection (AOI)

Limited value for BGA joints since they're hidden. AOI can verify:

• Package placement and orientation
• Peripheral ball visibility (partial view)
• Solder fillet at package edges

Electrical Testing

Boundary scan (JTAG): Tests connections through built-in test circuits. Detects opens and shorts without physical probing.

Functional testing: Verify the circuit works as designed. Catches defects that other methods miss.

BGA Rework

When things go wrong, BGAs can be removed and replaced:

Rework Station Setup

A dedicated BGA rework station includes:

• Top heater (infrared or hot air)
• Bottom preheater
• Precise temperature control with thermocouples
• Vacuum pickup for component removal
• Vision system for alignment

Pre-Bake Requirement

Moisture absorbed by the PCB or package causes popcorning during rework. Bake at 125°C for 24 hours before attempting removal.

Removal Process

1. Apply flux around the package perimeter
2. Preheat the bottom of the board (150-180°C)
3. Heat the top with controlled ramp rate
4. Monitor temperature with thermocouples
5. When solder melts, lift the package with vacuum

Site Preparation

1. Remove residual solder using wick or specialized tools
2. Clean the site with isopropyl alcohol
3. Inspect pads for damage
4. Apply fresh flux

Reballing (If Needed)

If the removed BGA is good but needs new balls:

1. Clean the package thoroughly
2. Apply flux using a stencil
3. Place new solder balls (using stencil or preformed array)
4. Reflow to attach balls

Replacement

1. Apply flux or solder paste to the site
2. Align the new BGA using vision system
3. Place the component
4. Reflow with controlled profile
5. Inspect with X-ray

Design for Manufacturability

Good BGA assembly starts with good design:

Pad design: Use NSMD (non-solder mask defined) pads for better joint shape. Match pad size to ball size.

Via placement: Keep vias away from pads or use filled/capped via-in-pad. Open vias cause solder wicking.

Fiducials: Include local fiducials near large BGAs for accurate placement.

Test points: Design in test access for boundary scan and functional testing.

Thermal relief: Thermal pads need proper via patterns for heat dissipation.

Conclusion

BGA assembly demands attention to every detail—from stencil design through inspection. The hidden nature of BGA joints means you can't rely on visual inspection; proper process control and appropriate inspection methods are essential.

With the right equipment, trained operators, and documented processes, BGA assembly becomes routine. Without them, it's a recipe for expensive rework and reliability problems. Invest in the process, and those tiny solder balls will serve you reliably for years.