1.27 Solder & Flux Storage

Solder paste and flux are unique: they are among the most time-sensitive materials in the factory, and their stability directly sets the stage for clean prints and reliable solder joints. Unlike static or moisture, their quality constantly degrades over time and temperature. Disciplined handling—from cold chain to thaw protocol—is essential to keep these chemistries predictable contributors to yield.

1.27.1 The Cold Chain and Storage Standards

The chemistry storage rules prioritize controlling the rate of material change through temperature and managing inventory based on its remaining life.

Storage Map: The Control Zones

Material	Where it Lives	Temperature	Key Storage Rule
Solder Paste (Jar/Cartridge)	CHEM-COLD Fridge	Per label (typically 0 – 10 ˚C)	Must be stored sealed and upright. Do not freeze unless specified. Cabinets must be temperature logged.
Liquid Flux (Wave/Selective)	Cool Cabinet or CHEM-COLD	15 – 25 ˚C unless specified	Keep caps tight; store out of sunlight.
Adhesives/Coatings	CHEM-COLD or Cool Cabinet	Per label	Respect pot life; track catalyzed mixes separately.
Solder Wire & Bars	CMP-STORE (Ambient)	Room	Wire flux-core has shelf life; keep spools bagged between uses.

Issuing Logic: FEFO Beats FIFO

For all chemistries (paste, flux, adhesives), the issue rule is FEFO = First Expiry, First Out.

• Priority: Kitting must pick the lot with the soonest expiry date that still has enough life for the planned build plus a buffer.
• MES Link: The MES/ERP must display the expiry date and days remaining during the pick process. Never override this without Quality Engineering (QE) sign-off.

1.27.2 Thaw-to-Print Protocol

The thermal transition from cold storage to the printing or soldering process is the most fragile step. Condensation is the primary risk.

Thawing Rules (The No-Condensation Zone)

1. Move Sealed: Transfer the jar or cartridge from CHEM-COLD to the ambient CMP-STORE (or kitting area) while sealed in its container.
2. Thaw Time: Allow the material to reach full room temperature per the manufacturer's specification (typical 2 – 6 hours for jars). Do not rush with forced heat, warm water, or a heat gun.
3. Condensation Check: The container must be opened only after it has reached room temperature. If the container sweats when opened, you waited too long; condensation has ruined the paste's performance.

Mixing and Readiness

• Mixing: Paste must be gentle; use a hand tumble/figure-8 motion for a jar, or knead a cartridge. Avoid whipping or introducing air.
• Final Check: Before printing, the paste must look smooth, uniform, and free of separation or granules. The first printed board must pass the SPI (Solder Paste Inspection) check for volume and height. If the print looks wrong, hold the line and call QE.

1.27.3 In-Use Limits and Expiry Management

Once opened, chemistries are actively degrading. Managing time and preventing contamination are the final control steps.

Open Time and Reuse

• Open Time on Line: Solder paste has a defined open time limit on the stencil (typically 4 – 8 hours). This must be tracked with a bench timer or the MES.
• Refresh Policy: Maintain consistency by adding small increments of fresh paste to the roll/pot regularly, rather than piling fresh on top of old paste all day. Retire the paste when its print quality (graping, smearing, SPI drift) becomes unstable.
• Return Ban: Never return opened paste to the fridge. Keep the "IN USE" paste at room temp for its site-defined use life (e.g., 24-48 hours), then scrap it. Mark reusable jars/cartridges with the number of re-opens (e.g., max 2 re-opens permitted).

Expiry and Disposition

• Near Expiry: Prioritize the lot using FEFO. Schedule builds to consume the material before the date.
• Expired: If the date passes, HOLD the lot. QE may approve a single re-qualification (e.g., a small wetting/solder balling test) or a vendor CoA extension. If not qualified the same day, the material must be scrapped. Do not trial expired chemistry on customer product.

1.27.4 Clean Handling and Common Traps

Preventing contamination and dilution is essential for predictable results.

Trap	Symptom	First Move (The Reliable Fix)
Opening Cold Jar	Condensation ruins the paste; water in the jar.	No forced heat. Thaw sealed at room temp only. Scrap if condensation occurs.
Diluting Flux	Weak activation, solder skips, or poor wetting.	No dilution unless explicitly defined in procedure; all mixes must be logged.
Returning Open Jars	Condensation ruins the paste the next day.	Keep "IN USE" paste at room temp until end of life; Scrap at limit, never return to the fridge.
Mixing Lots	Inconsistent print quality across the run.	Use one lot per line/printer; requires a full stencil wipe and changeover if the lot changes mid-run.
Contamination	Paste degradation, metal filings, or graping.	Use dedicated spatulas and lint-free wipes. Cap containers immediately after dispensing.

Final Note on Traceability: Each container's UID is tied to the lot, expiry, thaw/open times, and disposition in the MES. This allows you to pull product built with a failing lot if a defect trend appears later.