5.5 Solder & Flux Storage

~~Cold-chain, thaw/mix windows, open time, and FEFO rules for chemistries.~~

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yield.

5.5.1 Purpose & scope

Keep chemistries—solder paste, liquid flux, adhesives/coatings if used—inside their safe window from dock to print/wave. This covers storage temps, FEFO issuing, thaw/mix, in-use limits, and clear ready/not-ready checks. It ties to 5.1 (IDs), 5.2 (area controls), and 5.6 (kitting/FEFO).

5.5.2 Storage map (cold chain & friends)

Material	Where it lives	Temperature	Notes
Solder paste (jar/cartridge)	CHEM-COLD fridge	Per label (typically 0–10 °C)	Keep sealed & upright. Do not freeze unless label says so. Separate from food/MSDs.
Liquid flux (wave/selective)	Cool cabinet or CHEM-COLD if specified	15–25 °C unless label says otherwise	Keep caps tight; avoid sunlight; no decanting to “mystery” bottles.
Adhesives/coatings (if used)	CHEM-COLD or cool cabinet	Per label	Respect pot life/UV sensitivity. Track catalyzed mixes separately.
Solder wire & bars	CMP-STORE (dry, clean)	Room	Flux-core wire has shelf life; keep spools bagged between uses. Bars: avoid oil/grease contamination.

Logging: All fridges/cabinets are logged; excursions get a note and a disposition check.

5.5.3 FEFO beats FIFO (issuing rules)

FEFO = First Expiry, First Out for paste, flux, adhesives/coatings.
FIFO remains fine for metals (wire/bar) unless the flux core has an expiry.
Kitting picks the soonest-expiring lot that still has enough life for the planned use + buffer.
MES shows expiry date and days remaining at pick; don’t override without QE sign-off.

5.5.4 Thaw & mix (make it printable, not sweaty)

Solder paste

Plan: Move from CHEM-COLD to the CMP-STORE (controlled room) while sealed.
Thaw sealed: Bring to room temp per jar size (typical 2–6 h; cartridges often 1–2 h). Don’t rush with heat or warm water.
Dew check: If the jar sweats, you opened too soon. Wipe dry; wait longer. Never open cold paste—condensation ruins it.
Mix:
- Jar: Gentle hand tumble/figure-8 1–2 minutes after thaw; avoid whipping air.
- Cartridge: Knead to uniform; install piston cleanly.
Label “IN USE” with Date/Time opened and Operator.

Liquid flux

Bring to use temp (label). Do not dilute unless procedure specifies; if you must, mix with documented ratios and label MIXED ON / USE BY.

5.5.5 Ready/not-ready checks (before you print/solder)

Paste on printer

Temperature: Room-temp by IR thermometer or the no-condensation test.
Viscosity/appearance: Smooth, no separation, no granules.
Roll height: Within site spec on the stencil (e.g., 8–12 mm).
Test print: First panel to SPI—volume/height within limits (see 7.x/9.x).

Flux at wave/selective

Visual: Clear/consistent color; no debris.
Flow: Spray/puddle pattern stable; no sputter.
Test coupon: Wetting pattern acceptable.

If any check fails → hold and call QE. Don’t “hope-print.”

5.5.6 In-use limits (open time & return rules)

Solder paste open time on printer: follow label/site spec (typical 4–8 h). Track with a bench timer or MES tile.
Refresh policy: Add small increments of fresh paste regularly; don’t pile old on old all day. Retire the roll if SPI drift or graping/smearing appears.
Jar/cartridge reuse: If re-capping, mark # of opens. Site rule example: max 2 re-opens, then scrap.
Never return opened paste to the fridge. Keep “IN USE” at room temp for the site-defined use life (e.g., 24–48 h), then scrap.
Flux pot/tank life: Follow vendor/site interval; log top-ups and filter changes.
Adhesives/coatings: Respect pot life and UV/moisture sensitivity; discard at USE BY.

5.5.7 Expiry approaching or passed (what to do)

Approaching: Prioritize by FEFO; schedule builds to consume.
Just past expiry: Hold the lot. QE may approve re-qualification (small slump/solder balling/wetting checks or vendor CoA extension). If not requalified the same day, scrap.
Well past expiry or unknown storage: Scrap—don’t trial on customer product.

5.5.8 Clean handling (keep chemistry clean)

Use dedicated spatulas and lint-free wipes; no metal filings, oil, or hand lotion.
Keep jar rims/cartridge tips clean; cap immediately after dispensing.
Don’t leave containers open on benches.
Segregate lead-free vs SnPb tools and containers; color-code.

5.5.9 Traceability (tie chemistry to the build)

Each container has a UID (5.1) tied to lot, expiry, thaw/open times, re-opens, and disposition.
Printers/wave machines log which lot ran each work order.
If a defect trend appears (e.g., graping), you can pull by lot and review 5.5.5 checks.

5.5.10 Acceptance cues (fast table)

Area	Accept	Reject
Cold chain	Fridge at spec; temps logged	No log; long excursions without action
Thaw	Opened only at room-temp; no condensation	Jar opened cold; water in paste
Mix	Smooth paste; cartridge kneaded; labeled IN USE	Lumpy/separated; unlabeled
Open time	Timer running; SPI stable	Guessing; SPI drift/bridging ignored
FEFO issue	Soonest expiry picked; enough life for run	Grabbed the newest; run ends with expired paste
Return rules	Opened paste not back to fridge	Re-chilled open jar “for tomorrow”

5.5.11 Common traps → smallest reliable fix

Trap	Symptom	First move
“Warm it faster” with heat gun	Dry edges, poor wetting	No forced heat; thaw sealed at room temp
Diluting flux “a little”	Weak activation, solder skips	No dilution unless procedure; log mixes
Never-ending paste roll	Graping, bridges by noon	Timebox open time; refresh or retire roll
Open jar to fridge	Condensation next day	Keep “IN USE” at room temp; scrap at limit
Mixing lots on one stencil	Inconsistent print	One lot per line; changeover + wipe if changing
No FEFO	Expired jar mid-run	FEFO in kitting; MES warns on pick

5.5.12 Pocket checklists

Before issuing

FEFO checked; enough life for planned run + buffer
Fridge/cabinet logs OK; no excursions

Thaw & mix

Thawed sealed to room; no condensation
Mixed/kneaded; jar/cartridge IN USE labeled

On the printer / wave

Open-time timer running
First article SPI/wet-out OK
Add fresh paste/flux per plan; retire when drift appears

End of shift

Scrap or reseal per rule; no fridge return for opened paste
Tanks capped; lots logged; bench wiped

~~Bottom~~

By line:storing treatcold, chemistriesthawing likecarefully, milk with a mission—keep them cold till needed, warm themmixing gently, useand themscrapping withinon thetime, clock,solder paste and issueflux byremain consistent allies instead of hidden risks. Following FEFO. Do that, and print/waveclear startready/not-ready clean,rules defectskeeps stay low,print and youwave neverprocesses discoverstable, “expired”predictable, halfwayand through a build.
defect-free.