4.2 Process Specs: Alloys, Stencils & Chemistry

The chemical ecosystem of the PCBA process — solder alloy, flux, and cleaning agents — determines the long-term reliability and electrochemical stability of the product. This section defines the standard "House Process" parameters.

Solder Alloy Standards

• SAC305 (Sn96.5/Ag3.0/Cu0.5): The industry standard Lead-Free alloy. Melting point: 217 ˚C. Peak Reflow Temp: 245 ˚C - 255 ˚C.
• SnPb (63/37): Leaded solder. Restricted to exempted industries (Mil/Aero/Medical). Warning: Mixing Lead-Free components (SAC balls on BGAs) with SnPb paste creates a "Mixed Metallurgy" joint that requires a specialized reflow profile to prevent segregation and cracking.

Flux and Cleanliness

• No-Clean (NC): The standard flux type. Residue is benign and non-conductive. Cleaning is not required.
• Water Soluble (WS): Aggressive activity for difficult soldering. Mandatory Wash is required.
• Cleanliness Spec: For WS processes, the PCBA must pass Ionic Contamination testing (ROSE test) with a limit of < 1.56 micrograms/cm² NaCl equivalent (per IPC-TM-650).

Stencil Design

The stencil controls the volume of solder paste.

• Aspect Ratio: Aperture width / Foil thickness > 1.5 to ensure paste release.
• Reductions: Fine-pitch apertures are reduced by 10-15% relative to the pad size to prevent bridging.
• Windowing: Large ground pads must be "window-paned" (segmented) to prevent excessive solder volume from floating the component.

Final Checklist