2.4 Process Specs: Alloys, Stencils and Chemistry

Soldering is not just the act of applying conductive glue; it is a complex metallurgical process based on phase changes, fluid dynamics, and surface tension. If the baseline chemistry is not properly controlled, the joint will eventually fail, even if the SMT machine placed the component perfectly.

This chapter defines the boundary between the EMS provider’s standard “House” parameters (which are fixed) and the Product-Specific parameters (which you must control) to ensure the metallurgical integrity of every joint.

The “House” Standard: Paste & Powder

Solder paste is a highly perishable, carefully tuned chemical mixture of suspended metal spheres and active flux. You should not rely on EMS “default” selections without first verifying that the particle size matches the microscopic geometry of your specific components.

1. Alloy Selection Logic

• SAC305 (Sn96.5/Ag3.0/Cu0.5): The standard industry default for Lead-Free assembly. It possesses a high melting point (~217˚C) and forms highly reliable intermetallic layers.
• SnPb (63/37): Reserved for Legacy or Mil-Aero products that demand specific reliability parameters. This requires segregated factory lines to prevent lead cross-contamination.
• Low-Temp (SnBi): Use ONLY if specific components (e.g., PET screens, cheap plastics) cannot withstand a 240˚C peak reflow temperature. The Risk: Bismuth generates brittle joints with very poor drop-shock resistance.

2. Powder Type (Particle Size)

You must select the powder type based on the “5-Ball Rule”: a minimum of 5 solder spheres must fit simultaneously across the narrowest stencil aperture width to ensure reliable paste release.

• Type 3 (25 – 45 µm): Obsolete. Allow only for very coarse, low-tech designs (0805+, 1.0mm BGA). It is inadequate for modern fine-pitch components.
• Type 4 (20 – 38 µm): The Standard. Require Type 4 for all general assemblies down to 0.5mm pitch (0402/QFN).
• Type 5 (15 – 25 µm): Specialist Operations Only. Required for ultra-dense 01005 chips or 0.3mm CSPs.
  • The Constraint: Type 5 has an exponentially higher surface-area-to-volume ratio and oxidizes rapidly. When Type 5 is used, you must reduce stencil life limits (e.g., require disposal after only 4 hours on the stencil).

3. The Handling Rules

• Thaw Time: Paste jars must stabilize at ambient factory temperature for a minimum of 4 hours before the seal is broken. Opening cold paste invites microscopic moisture condensation, which can lead to solder balling defects during reflow.
• Stencil Life: Avoid allowing active paste to sit dormant on the stencil beyond 8 hours.

Stencil Engineering: The Variable

The stainless steel stencil is the primary volumetric control valve for the entire SMT process. Over 60% of all SMT defects originate at the printing stage. Do not simply accept a generic, factory-wide “Global Thickness.”

1. Foil Thickness & Stepped Technology

• Standard Foil: 100 µm (4 mil) or 127 µm (5 mil).
  • When the board is predominantly fine pitch (< 0.5mm), default to 100 µm.
  • When the board features heavily mixed technology (Heavy Power Connectors + Fine Pitch BGAs), you must require a Stepped Stencil.
    • Step-Down: locally laser-etch the thickness (e.g., 127 µm → 100 µm) exclusively for the BGA zone to prevent bridging.
    • Step-Up: locally increase the thickness specifically for Through-Hole Reflow (Pin-in-Paste) or heavy RF shield cans to provide adequate solder volume.

2. Area Ratio (Release Physics)

Paste is designed to adhere to the copper pad, not the microscopic walls of the stencil aperture.

• The Math: Area Ratio = (Area of Aperture Opening) / (Area of Aperture Walls).
• The Target Threshold: The calculated ratio must be ≥ 0.66.
• The Remediation: If the math result is too low, the EMS must use Nano-Coating or explicitly Electroformed Nickel stencils to lubricate the paste release.

3. Aperture Geometry Engineering

• General Pads: Symmetrically reduce 1:1 apertures by 10% volume to prevent peripheral solder balls.
• Center Grounds (QFN/BTC/Power Pads): Reduce paste coverage to 50 – 60% using a segmented “Window Pane” pattern.
  • The Logic: 100% paste coverage hydraulically lifts the component body (“Beaching”), disconnecting the fragile perimeter signaling pins from their pads.

Reflow Profile Ownership

Accepting a “generic” oven profile from the EMS is a risk. Every unique PCBA mass requires a validated, instrumented thermal recipe.

The Profile Constraints:

• Soak Zone: Chemically activates the flux. Too long = “Graping” (dried paste); Too short = Severe Voiding trapped by rapid outgassing.
• Time Above Liquidus (TAL): Target exactly 60 – 90 seconds above 217˚C (for SAC305).
• Peak Temp: Maintain 235˚C – 245˚C.
• The Delta-T Rule: The physical temperature difference between the coldest joint (under a massive BGA) and the hottest joint (an isolated 0402 component) must be ≤ 10˚C at the peak of the reflow cycle.

Chemistry & Cleaning Discipline

Clearly defining how to clean the board is equally as critical as defining what to clean it with.

1. Flux Chemistry

• No-Clean (ROL0/ROL1): The modern standard. The baked-on resin residue is chemically benign and safely encapsulates active ions.
• Water Soluble (OA): Highly aggressive, acidic chemistry. It must be washed off without exception.

2. The Cleaning Agents

• When washing a No-Clean flux (for conformal coating surface prep), require a specifically engineered Saponifier chemistry (e.g., Kyzen, Zestron).
  • The Warning: Never use pure IPA (Isopropyl Alcohol) or water alone for No-Clean wash. IPA is chemically incapable of dissolving flux salts; it merely hydrates and smears them, leaving a highly problematic white residue (“White Haze”).
• The Process Window: If utilizing Water Soluble flux, the wash cycle must initiate within < 4 hours of exiting the reflow oven to prevent microscopic trace corrosion.

Final Baseline Checklist

The Control Point	The Operational Requirement
Powder Type Matrix	Type 4 (Baseline Standard) vs Type 5 (Fine Pitch only).
Area Ratio Math	≥ 0.66 (Mandatory for acceptable geometric paste release).
Stencil Thickness	100 µm (Fine Pitch dominance) or 127 µm (Standard dominance).
Stepped Stencil	Mandatory if component height/volume mix creates a physical conflict.
QFN Ground Aperture	50-60% volumetric coverage via a Window Pane pattern.
Reflow TAL Timing	60 – 90 seconds maintained (SAC305).
Cleaning Agent	Require a Saponifier for No-Clean wash; Ban pure IPA.
Paste Thaw Time	4 hours ambient thermal stabilization before breaking the jar seal.