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2.3 Shelf Life & Expiry Control: Chemical Integrity

Electronics manufacturing relies on chemistry as much as physics. Solder paste, Anisotropic Conductive Film (ACF), conformal coatings, and potting compounds are reactive polymers. From the moment of manufacture, these materials degrade. "Shelf Life" defines the window of chemical reliability. Utilization of expired or mishandled material rarely results in immediate line fallout; instead, it generates latent field failures — dendritic growth, delamination, and brittle joints — that manifest months after deployment. The "Cold Chain" and the "Time Clock" must be enforced with absolute rigidity.

Incoming Quality (IQC): The Cold Chain

Material validation begins at the receiving dock. Transport conditions often violate storage specifications.

Verification Logic:

  • If material requires refrigeration (2°C – 10°C) and arrives without a data logger -> Then Quarantine immediately. Adherence to temperature limits during transit cannot be verified.
  • If remaining shelf life is < 25% -> Then Reject or flag for "Urgent Priority." Stocking material that will expire before utilization increases scrap costs and risk.
  • Action: Apply an internal trace label: "RECEIPT DATE" and "EXPIRY DATE". Supplier packaging labels are often illegible or obscured during storage.

Storage Discipline

Refrigeration equipment acts as a critical process chamber.

Control Parameters:

  • Temperature Monitoring: Continuous (24/7) data logging is mandatory. Manual daily checks are insufficient. Alarm thresholds must be set (e.g., Low < 2°C / High > 10°C).
  • Power Failure Protocol:
    • If power loss duration > 4 hours -> Then Quarantine all contents.
    • Disposition: Engineering must calculate cumulative thermal exposure. If unknown, material must be scrapped.
  • FIFO (First-In-First-Out): Inventory management must be driven by Expiry Date, not Receipt Date. Oldest material is positioned at the front.

Usage Protocol: Thawing and Pot Life

The transition from controlled storage to the production environment introduces the highest risk of degradation.

The Thawing Rule (Thermodynamics)

Opening cold containers in an ambient environment causes immediate condensation. Moisture absorption leads to solder balling (popcorning) and voiding during reflow.

  • Rule: Material must stabilize to ambient temperature before the seal is broken.
  • Standard Times:
    • Solder Paste (500g Jar): 4 – 8 hours.
    • Adhesives (30cc Syringe): 2 – 4 hours.
  • Control: A "Thaw Start Time" label must be applied upon removal from refrigeration. Infrared (IR) temperature verification is recommended before opening.

Pot Life vs. Shelf Life

  • Shelf Life: Viability within the sealed, original container.
  • Pot Life (Working Life): Viability after exposure to the environment (breaking the seal or mixing components).
    • Example: Two-part epoxy may have a 1-year shelf life but a 45-minute pot life. Beyond 45 minutes, viscosity shifts invalidate the wetting capability.
  • Stencil Life: Solder paste residing on the stencil degrades due to oxidation and flux evaporation. Prohibition: Never scrape used paste from the stencil back into the fresh jar. This cross-contaminates the reliable chemistry.

Expiry Management & Re-Certification

Expired material is non-conforming by definition.

The Expiry Logic:

  • If material is Expired -> Then The ERP system must block transaction codes, or physical segregation (Red Bin) must be enforced.
  • If production is critical and no fresh stock exists -> Then Engineering may authorize a "Life Extension" via a formal Deviation.

Re-Certification Protocol:

Extension requires empirical proof of chemical viability. Sticker updates without testing are negligence.

  1. Solder Paste: Perform IPC-TM-650 Solder Ball Test and Wetting Test.
  2. Adhesives: Perform shear strength testing on a sample coupon.
  3. Disposition:
    • Pass: Extend expiry by short duration (e.g., 2 weeks). Label as "RE-VALIDATED".
    • Fail: Scrap immediately.

Final Checklist

Control Point

Critical Requirement

Cold Chain Integrity

Incoming shipments must include Temperature Data Loggers.

FIFO Discipline

Selection based strictly on Earliest Expiry Date.

Thawing Control

Do Not Open seal until container reaches room temperature.

Traceability

Production records must link Material Lot # to PCB Serial #.

Contamination

Used paste/adhesive must never be mixed with Fresh stock.

Environment

Production floor RH/Temp must be within the material's Pot Life specifications.

Disposal

Expired chemical waste is Hazardous. Disposal requires specific compliance documentation.


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