1.1 Paste Chemistry & Alloy Choice

Solder paste is the single largest variable in the SMT process, responsible for over 65% of end-of-line defects. It is not merely "glue"; it is a complex rheological system that must withstand printing shear forces, hold components during transport, and chemically react to form a metallurgical bond during reflow. Selecting the wrong chemistry guarantees unstable printing, voiding, or latent field failures, regardless of how expensive your printer or oven is. Treat paste selection as an engineering specification, not a consumable commodity.

Flux System Selection: The Cleanliness Trade-Off

The flux vehicle dictates the process window, voiding performance, and residue risks. Choose the flux system based on the cleaning capability and reliability requirements of the final assembly.

Scenario A: High-Reliability, Dense Assembly (Aerospace, Medical, High-Density Digital)

Requirement: Zero residue risk, maximum wetting on oxidized pads.

Action: Select Water-Soluble (WS) paste.

• Why: WS fluxes contain aggressive activators (organic acids) that clean oxides rapidly. They provide the widest wetting window and lowest voiding rates.
• The Engineering Risk: The residue is corrosive. If the washing process fails (water temperature, pressure, or saponifier concentration drop), the remaining ionic contamination will cause dendritic growth and shorts.
• Mandatory Control: You must have an in-line cleaner and perform daily Ionic Contamination testing (ROSE/SEC) to verify cleaning efficacy.

Scenario B: Standard Industrial/Consumer (IoT, Control Boards, LED)

Requirement: Cost efficiency, no cleaning process.

Action: Select No-Clean (NC) paste (ROL0/ROL1).

• Why: The residue is engineered to be non-conductive and benign after reflow. It eliminates the capital and operating expense of a wash line.
• The Engineering Risk: "No-Clean" does not mean "Invisible."
  • If Reflow is too Cold: Activators remain unreacted and conductive -> Leakage currents.
  • If Reflow is too Hot: Residue chars -> Interferes with In-Circuit Test (ICT) probes (false failures).
• Mandatory Control: Verify flux compatibility with conformal coating if used. Validate ICT probeability.

Alloy Selection: Thermal & Mechanical Integrity

The alloy defines the melting point and the mechanical fatigue resistance of the joint. Do not default to "Standard SAC305" without verifying the application constraints.

Standard Application: SAC305 (Sn96.5 / Ag3.0 / Cu0.5)

• Melting Point: ≈ 217˚C – 220˚C.
• Usage: The industry default for general SMT.
• Outcome: Provides acceptable thermal cycling performance for consumer electronics (0˚C to 60˚C operational range).
• Limit: Avoid for high-stress automotive or aerospace applications where thermal shock exceeds -40˚C to +125˚C.

High-Reliability Application: Doped Alloys (SAC-Q, SAC-I, SnNi)

• Usage: Automotive under-hood, ruggedized industrial.
• Mechanism: Dopants like Bismuth (Bi), Nickel (Ni), or Antimony (Sb) pin the grain boundaries of the solder structure.
• Outcome: Prevents micro-cracks propagation during thermal cycling. measurable 2x – 3x increase in drop-shock and thermal fatigue life compared to standard SAC305.

Low-Temperature Application: SnBi (Tin-Bismuth)

• Melting Point: ≈ 138˚C.
• Usage: Heat-sensitive components (cheap LEDs, PET flex circuits).
• Risk: The joint is brittle.
• Engineering Consequence: Mechanical shear strength is < 50% of SAC305. Boards must not be subjected to drop shock or bending. Never mix SnBi paste with SAC305 component balls unless specifically profiled to fully mix the alloy, or the joint will fail catastrophically.

Pro-Tip: If your Voiding Rate on QFN thermal pads exceeds 25% (IPC Class 3 failure), do not just tweak the profile. Switch to a "Low-Voiding" flux formulation specifically engineered with different solvent outgassing rates. Chemistry fixes voiding better than profiling does.

Powder Size (Mesh) & Release Stability

Particle size controls the release of paste from the stencil aperture. Mismatched powder size leads to clogging, insufficiency, and rapid oxidation.

Logic for Powder Selection:

1. If Smallest Pitch ≥ 0.5 mm AND Smallest Aperture Width > 0.25 mm:
   • Use Type 3 (T3) (25 – 45 µm).
   • Benefit: Lower surface area oxide, longer stencil life (> 8 hours), lower cost.
2. If Smallest Pitch 0.4 mm OR Area Ratio (AR) is 0.60 – 0.66:
   • Use Type 4 (T4) (20 – 38 µm).
   • Reason: T3 particles will block the aperture. T4 is the modern standard for mixed-technology boards.
3. If Smallest Pitch ≤ 0.3 mm (01005s, µBGA):
   • Use Type 5 (T5) (15 – 25 µm).
   • Risk: High surface area leads to rapid flux exhaustion.
   • Consequence: Stencil life is reduced to < 4 hours. You must replenish paste more frequently to maintain tack and flux activity.

Pro-Tip: Never top up a jar of T4 paste with leftover T3. The mixed rheology will cause unpredictable rolling and release, destroying your Cp/Cpk.

Traceability & Incoming Control

You cannot control the process if you do not control the material. Every jar of paste must be tracked.

• Lot Number: Record and link to the Production Job ID. This is the only way to contain a defect if a bad batch is discovered later.
• Date of Manufacture (DOM): Enforce a strict shelf life (typically 6 months at 0–10˚C).
  • Action: If DOM > 6 months -> Dispose. Do not requalify. The flux activators degrade over time even in cold storage.
• Metal Load: Verify the weight percentage (e.g., 88.5%).
  • Drift: If metal load is too low (high flux content), slump and bridging increase. If too high, paste dries out and clogs.

Final Checklist

Parameter	Specification / Target	Control Method
Flux Classification	J-STD-004 (e.g., ROL0, ORL0)	Verify datasheet matches Wash/No-Clean strategy.
Halogen Content	Halogen-Free vs. Halogenated	Confirm customer environmental compliance (IEC 61249-2-21).
Alloy Melting Point	Specific to Application (e.g., 217˚C)	Confirm reflow oven capability (Max temp vs. component limits).
Powder Size	Type 3, 4, or 5	Must match stencil Area Ratio (Target AR ≥ 0.66).
Viscosity (Malcom)	e.g., 180 – 220 Pa·s	Incoming inspection or Certificate of Analysis (CoA) verification.
Shelf Life	< 6 months from DOM	FIFO (First-In-First-Out) physical stock rotation.
Stencil Life	> 8 hours (T3/T4) / > 4 hours (T5)	Production log monitoring (abandon if exceeded).