PCBA — SMT + Through-Hole Assembly

IPC-A-610 Class 2 vs. Class 3: Acceptance Criteria Differences

IPC-A-610 defines workmanship standards for assembled PCBs. The class determines what types of solder joint defects are acceptable (Class 2) vs. required for rejection (Class 3):

Class 2 (General Electronics Products). Allows minor solder bridges that do not violate minimum electrical clearance, limited voiding in solder joints (up to 25% void area for BGA balls), and certain cosmetic defects. Covers the large majority of consumer and industrial electronics.

Class 3 (High Performance Electronics). Zero tolerance for solder bridges, <25% void area by IPC-7095 for BGA, tighter lead protrusion requirements for through-hole, and requires certification of the inspection personnel (IPC-A-610 CIS/CIT qualified). Required for aerospace, medical, and safety-critical applications. Expect 20–40% longer inspection time and higher assembly cost per board.

Requesting Class 3 for a consumer product is common but often unnecessary and costly. Before specifying Class 3, check your actual end-product quality requirements. If you are selling on Amazon and your failure rate target is <0.5%, Class 2 with a rigorous first article inspection process is typically sufficient.

AOI vs. X-Ray for BGA Packages

AOI (Automated Optical Inspection). Camera-based inspection after reflow. Effective for detecting: missing components, wrong component orientation, tombstoning, solder bridges on visible leads, and incorrect part placement (via component outline and color recognition). Cannot see solder joints hidden under component bodies (BGA, QFN, LGA packages).

X-ray (2D or CT). Required for BGA and large QFN/LGA packages where solder joints are entirely hidden under the package body. 2D X-ray shows void percentage and gross solder bridging. CT (computed tomography / 3D X-ray) provides cross-sectional view for detailed void analysis. CT is significantly more expensive and slower — used for failure analysis, not 100% production inspection.

For boards with BGA components, require 100% 2D X-ray inspection on production runs. A sampling plan (e.g., first 10 boards + 1 per 50 boards thereafter) is acceptable for established processes with a strong SPC (Statistical Process Control) history.

Stencil Thickness and Paste Volume

Stencil thickness determines the paste deposit volume, which directly affects solder joint quality:

• 0.12mm stencil (standard): correct for most 0402+ components and QFP with 0.65mm pitch and larger. Produces adequate paste volume for reliable reflow joints.
• 0.10mm stencil: used for mixed boards with both 0201 components and larger passives on the same side. Reduces paste volume for small components while maintaining acceptable volume for larger pads.
• 0.08mm stencil: for 01005 components and 0.4mm-pitch QFP/CSP. Very thin paste deposits — requires optimized aperture ratios (≥0.66 area ratio) and regular stencil cleaning.

For boards with large power pads (e.g., QFN thermal pads), stencil aperture reduction (typically 50–75% of pad area, in a grid pattern) is required to prevent solder voiding and component floating during reflow. Confirm the factory’s stencil design rules address this — many do not by default.

First Article Inspection (FAI) Procedure

FAI verifies that the first production boards meet specification before the full production run proceeds. A robust FAI process includes:

1. Visual inspection against IPC-A-610 Class 2/3 criteria
2. Dimensional check on critical component placements (pick-and-place offset measurement)
3. AOI full scan with no deferred defects
4. X-ray inspection for all BGA/QFN components
5. Functional test using your test fixture or test firmware
6. ICT (In-Circuit Test) if applicable (boundary scan or bed-of-nails)

Require sign-off on FAI results before the factory proceeds past the first 5–10 boards. Finding a reflow profile issue at FAI is far cheaper than discovering it after 500 boards are assembled.