6.8 Solder Paste, Flux, Chemicals Storage & Handling

Solder paste and adhesives are not static components; they are chemically active mixtures suspended in a state of arrested reaction. From the moment they are manufactured, the flux seeks to react with the alloy, and the polymer seeks to cross-link. Poor storage accelerates this "aging," resulting in dry paste, poor wetting, and voiding during reflow. Simultaneously, these materials are often hazardous (flammable, corrosive, toxic). Storage must satisfy two masters: Process Stability (Physics) and EHS Compliance (Safety).

Cold Chain Governance (Paste & Adhesives)

Most solder pastes and pre-mixed epoxies require refrigeration (typically 2°C – 10°C) to retard chemical reaction.

• Dedicated Refrigeration: Use industrial-grade fridges with digital data logging. Do not use the "breakroom fridge."
  • Alarm Rule: If Temp > 10°C for > 60 minutes → Trigger Alarm.
  • Power Failure: If power is lost > 4 hours → Quarantine. Engineering must verify viscosity/flux activity before release.
• The "Thaw" Protocol:
  • Paste must stabilize to ambient temperature before opening.
  • Rule: Remove from fridge 4–8 hours before use (depending on jar size).
  • Do Not: Force heat (heater/warm water). This causes flux separation.
  • Do Not: Open the jar while cold. This causes condensation (water absorption) → "Slump" and "Solder Balls" in reflow.

Chemical Storage & Segregation (EHS Core)

Treat the chemical store as a hazard zone. Segregate based on Safety Data Sheet (SDS) Compatibility, not alphabetical order.

1. Flammables (IPA, Solvents, Conformal Coatings):
   • Store in yellow, fire-rated Metal Safety Cabinets.
   • Grounding: The cabinet must be grounded to Earth to prevent static buildup.
   • Ventilation: Ensure vents are unblocked (or piped to exhaust if required).
2. Corrosives (Flux Removers, Acids):
   • Store in non-metallic (poly/plastic) cabinets or trays to prevent corrosion of the shelf itself.
   • Segregation: Never store Acids (low pH) immediately above or next to Bases (high pH) or Flammables.
3. Secondary Containment:
   • Every liquid container must sit on a tray or bund capable of holding 110% of the volume of the largest container.
   • Logic: If a bottle leaks → Then the chemical stays in the tray, not on the floor/drain.

Operational Discipline: Issue, Pot Life & Returns

Chemicals expire in two ways: Shelf Life (unopened) and Pot Life (opened).

• Issue by Batch (FEFO):
  • Always pick the batch with the earliest Expiration Date.
  • If Expiration Date has passed → Block Issue. Do not "try it out."
• "Opened" Labeling:
  • The moment a seal is broken, the operator must apply a label: "Opened Date: [DD/MM] – Expire: [Time/Date]".
  • Pot Life Rule: If Pot Life is 24 hours → Then the material is Scrap at Hour 25, regardless of how much is left.
• Return-to-Fridge Rules:
  • Never mix used solder paste back into a jar of fresh paste. This contaminates the new batch with oxidized spheres and dried flux.
  • If returning used paste → Then put it in a separate jar labeled "USED" and consume it first on the next shift (if within Pot Life).

Final Checklist & Audit