2.4 Process Specs: Alloys, Stencils and Chemistry

Soldering is more than just applying conductive glue; it’s a complex metallurgical process governed by phase changes, fluid dynamics, and surface tension. If the foundational chemistry isn’t properly controlled, the solder joint will eventually fail, even if the SMT machine placed the component perfectly.

This chapter clarifies the boundary between the EMS provider’s fixed “House” parameters and the Product-Specific parameters you must define and control to ensure the metallurgical integrity of every joint.

The “House” Standard: Paste & Powder

Solder paste is a perishable, carefully tuned chemical mixture of suspended metal spheres and active flux. It’s essential to verify that the selected particle size matches the microscopic geometry of your specific components, rather than relying solely on the EMS provider’s default selections.

1. Alloy Selection Logic

SAC305 (Sn96.5/Ag3.0/Cu0.5): This is the standard industry default for lead-free assembly. It has a relatively high melting point (~217˚C) and forms highly reliable intermetallic layers.
SnPb (63/37): This alloy is typically reserved for legacy or military/aerospace products that demand specific reliability parameters. Its use requires segregated factory lines to prevent lead cross-contamination.
Low-Temperature Alloys (SnBi): Use these alloys only when specific components (such as PET screens or certain plastics) cannot withstand a standard 240˚C peak reflow temperature. Note: Bismuth-based alloys can form brittle joints with poor resistance to mechanical shock.

2. Powder Type (Particle Size)

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

Type 3 (25 – 45 µm): This type is now considered obsolete for modern designs. Its use should be limited to very coarse, low-density layouts (e.g., components 0805 or larger, 1.0mm pitch BGAs).
Type 4 (20 – 38 µm): This is the current standard. Specify Type 4 powder for all general assemblies, especially those with components down to 0.5mm pitch (like 0402 resistors or QFNs).
Type 5 (15 – 25 µm): This is a specialist grade. It is required for ultra-dense components like 01005 chips or 0.3mm pitch CSPs.
- Important Constraint: Type 5 powder has a much higher surface-area-to-volume ratio, which causes it to oxidize more rapidly. When Type 5 is specified, you must enforce stricter stencil life limits—for example, requiring paste disposal after just 4 hours on the stencil.

3. The Handling Rules

Thaw Time: Solder paste jars must be allowed to stabilize at ambient factory temperature for a minimum of 4 hours before the seal is broken. Opening a cold jar can cause microscopic moisture condensation inside, which may lead to solder balling defects during reflow.
Stencil Life: To maintain print quality and chemical activity, avoid letting solder paste sit dormant on the stencil for more than 8 hours.

Stencil Engineering: The Critical Variable

The stainless steel stencil acts as the primary volumetric control valve for the entire SMT process. Since over 60% of all SMT defects originate at the printing stage, Accepting a generic, factory-wide “Global Thickness” is prohibited.

1. Foil Thickness & Stepped Technology

Standard Foil: Common thicknesses are 100 µm (4 mil) or 127 µm (5 mil).
- For boards dominated by fine-pitch components (< 0.5mm), 100 µm should be the default choice.
- For boards with a heavily mixed technology load (such as combining heavy power connectors with fine-pitch BGAs), a Stepped Stencil should be specified.
  - A Step-Down area involves locally laser-etching the stencil to be thinner (e.g., from 127 µm to 100 µm) in the BGA zone to prevent solder bridging.
  - A Step-Up area involves locally increasing the thickness to provide extra solder volume for applications like Through-Hole Reflow (Pin-in-Paste) or under large RF shield cans.

2. Area Ratio (The Physics of Release)

Solder paste is designed to adhere to the copper pad, not the walls of the stencil aperture. The Area Ratio is a key metric for predicting clean paste release.

The Calculation: Area Ratio = (Area of the Aperture Opening) / (Area of the Aperture Walls).
The Target: The calculated ratio should be ≥ 0.66.
The Solution: If the calculated ratio is too low, the EMS provider should apply a Nano-Coating to the stencil or use Electroformed Nickel stencils to improve paste release characteristics.

3. Aperture Geometry Engineering

General Pads: For standard pads, use a 1:1 aperture but reduce its area by approximately 10%. This slight volumetric reduction helps prevent the formation of peripheral solder balls.
Center Ground Pads (QFN/BTC/Power Pads): For large thermal or ground pads, reduce the paste coverage to 50 – 60% by using a segmented “Window Pane” pattern.
- The Reason: Applying 100% paste coverage can create excessive hydraulic pressure during reflow, potentially lifting the component body (a defect known as “beaching”). This can disconnect the fragile perimeter signal pins from their pads.

Reflow Profile Ownership

Accepting a generic oven profile from the EMS provider introduces significant reliability risk. Every unique PCBA, due to its mass and component mix, requires a validated, instrumented thermal recipe.

The Profile Constraints:

Soak Zone: This zone chemically activates the flux. If the soak is too long, the paste can dry out (“graping”); if too short, rapid outgassing can lead to severe voiding.
Time Above Liquidus (TAL): For SAC305 alloy, the board should spend 60 – 90 seconds above its liquidus temperature of 217˚C.
Peak Temperature: The maximum temperature should be maintained between 235˚C and 245˚C.
The ΔT Rule: The temperature difference (ΔT) between the coldest joint (e.g., under a massive BGA) and the hottest joint (e.g., a small, isolated 0402 resistor) should be ≤ 10˚C at the peak of the reflow cycle.

Chemistry & Cleaning Discipline

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

1. Flux Chemistry

No-Clean (ROL0/ROL1): This is the modern standard. The baked-on resin residue is chemically benign and safely encapsulates any active ions.
Water-Soluble (OA): This is a highly active, acidic chemistry. It must be thoroughly washed off the assembly after reflow.

2. The Cleaning Agents

When washing a board assembled with No-Clean flux (typically for surface preparation before conformal coating), you must require a specifically engineered Saponifier chemistry (from suppliers like Kyzen or Zestron).
Warning
- Using pure Isopropyl Alcohol (IPA) or water alone to clean No-Clean flux is ineffective. IPA cannot dissolve the flux residues; it merely hydrates and smears them, leaving a problematic white residue often called “White Haze.”
For Water-Soluble flux, boards must be washed within 4 hours of exiting the reflow oven to prevent microscopic trace corrosion.

Recap: Alloy, Powder, Stencil, and Process Selection

Parameter	Requirement	Value / Condition	Action / Constraint
Alloy	Selection Logic	SAC305 (Sn96.5/Ag3.0/Cu0.5) – Standard	SnPb – Segregated lines only. SnBi – For temp-sensitive parts only.
Powder Type	Based on “5-Ball Rule”	Type 4 (20–38 µm) – Standard	Type 5 (15–25 µm) – For 01005 / <0.3mm pitch. Enforce ≤4h stencil life.
Stencil Thickness	Adaptive to component mix	100 µm – Default for <0.5mm pitch	Stepped stencil required for mixed heavy/fine-pitch technology.
Aperture Design	Optimize paste volume	General pads: 1:1 aperture, -10% area. Large ground pads: 50–60% coverage.	Use “Window Pane” pattern for large pads to prevent beaching.
Reflow Profile	Validated per unique PCBA	TAL (SAC305): 60–90s >217°C. Peak Temp: 235–245°C. ΔT across board: ≤10°C.	Use of generic EMS profile is prohibited.
Cleaning	Chemistry & process discipline	No-Clean flux: Use saponifier, not IPA/water. Water-Soluble flux: Wash within 4h of reflow.	Define cleaning agent and process window.