2.4 Process Specs: Alloys, Stencils and Chemistry

Soldering is not applying conductive glue; it is a complex metallurgical process relying on phase changes and surface tension. If the chemistry is uncontrolled, the joint will fail, regardless of placement accuracy. This chapter defines the boundary between the EMS "House Standard" (fixed) and the Product-Specific parameters (variable) to secure the integrity of every joint.

The "House" Standard: Paste & Powder

Solder paste is a perishable chemical mixture. Do not rely on "default" selections without verifying the particle size against the component geometry.

1. Alloy Selection

• SAC305 (Sn96.5/Ag3.0/Cu0.5): The industry default for Lead-Free. High melting point (~217˚C).
• SnPb (63/37): Legacy/Mil-Aero only. Requires segregated lines to prevent lead contamination.
• Low-Temp (SnBi): Use only if components (e.g., PET screens) cannot withstand 240˚C. Risk: Brittle joints with poor drop-shock resistance.

2. Powder Type (Particle Size)

Select the powder type based on the "5-Ball Rule": at least 5 solder spheres must fit across the smallest aperture width to ensure release.

• Type 3 (25 – 45 µm): Legacy. Use only for coarse designs (0805+, 1.0mm BGA). Inadequate for modern fine pitch.
• Type 4 (20 – 38 µm): The Standard. Mandate Type 4 for all general assemblies down to 0.5mm pitch (0402/QFN).
• Type 5 (15 – 25 µm): Specialist Only. Required for 01005 chips or 0.3mm CSPs.
  • Constraint: Type 5 has high surface area and oxidizes rapidly. If Type 5 is used, Then reduce stencil life limits (e.g., discard after 4 hours).

3. Handling Rules

• Thaw Time: Paste must stabilize at ambient temperature for 4 hours before opening. Opening cold paste invites moisture condensation (solder balling).
• Stencil Life: Do not leave paste on the stencil beyond 8 hours.

Stencil Engineering: The Variable

The stencil is the primary volume control valve. 60% of defects originate here. Do not use a generic "Global Thickness."

1. Foil Thickness & Stepped Technology

• Standard Foil: 100 µm (4 mil) or 127 µm (5 mil).
  • If the board has predominantly fine pitch (< 0.5mm), Then default to 100 µm.
  • If the board is mixed (Heavy Connectors + Fine Pitch BGA), Then require a Stepped Stencil.
    • Step-Down: Locally reduce thickness (e.g., 127 µm → 100 µm) for the BGA area.
    • Step-Up: Locally increase thickness for Through-Hole Reflow or heavy shield cans.

2. Area Ratio (Release Physics)

Paste must adhere to the pad, not the stencil wall.

• Formula: Area Ratio = (Area of Aperture Opening) / (Area of Aperture Walls).
• Threshold: Ratio must be ≥ 0.66.
• Remediation: If the ratio is failing, use Nano-Coating or Nickel-etched stencils to improve release.

3. Aperture Design

• General Pads: Reduce 1:1 apertures by 10% to prevent solder balls.
• Center Grounds (QFN/BTC): Reduce coverage to 50 – 60% using a "Window Pane" pattern.
  • Reasoning: 100% paste coverage lifts the component ("Beaching"), disconnecting the perimeter pins.

Reflow Profile Ownership

A "generic" profile is a gamble. Every unique PCBA requires a validated thermal recipe.

Profile Constraints:

• Soak Zone: Activates flux. Too long = "Graping" (dried paste); Too short = Voiding.
• Time Above Liquidus (TAL): Target 60 – 90 seconds above 217˚C (SAC305).
• Peak Temp: 235˚C – 245˚C.
• Delta-T: The temperature difference between the coolest joint (large BGA) and hottest joint (small resistor) must be ≤ 10˚C at peak.

Chemistry & Cleaning Discipline

Defining how to clean is as important as what to clean.

1. Flux Chemistry

• No-Clean (ROL0/ROL1): The standard. Residue is benign.
• Water Soluble (OA): Highly active. Must be washed.

2. Cleaning Agents

• If cleaning No-Clean flux (for conformal coating prep), Then use a specialized Saponifier (e.g., Kyzen, Zestron).
  • Warning: Do NOT use pure IPA (Isopropyl Alcohol) or water alone. IPA does not dissolve flux salts; it merely spreads them, leaving a conductive white residue ("White Haze").
• Process Window: If using Water Soluble flux, wash within < 4 hours of reflow to prevent trace corrosion.

Final Checklist

Control Point	Critical Requirement
Powder Type	Type 4 (Standard) vs Type 5 (Fine Pitch only).
Area Ratio	≥ 0.66 (Mandatory for release).
Stencil Thickness	100 µm (Fine Pitch) or 127 µm (Standard).
Stepped Stencil	Mandatory if component mix creates volume conflict.
QFN Aperture	50-60% coverage (Window Pane).
Reflow TAL	60 – 90 seconds (SAC305).
Cleaning Agent	Use Saponifier for No-Clean; Ban pure IPA.
Thaw Time	4 hours stabilization before seal break.