5.5 Solder & Flux Storage

Solder paste and flux are among the most time-sensitive materials in electronics manufacturing, and their stability directly sets the stage for clean prints and reliable joints. Unlike metals or boards, these chemistries keep changing from the moment they leave controlled storage, making the cold chain, thawing, and mixing as critical as any machine setting. Open time defines their usable life on the line, and FEFO issuing ensures the right lot runs out on schedule rather than mid-build. With disciplined handling, consumables shift from being unpredictable troublemakers to steady, repeatable contributors to 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

By storing cold, thawing carefully, mixing gently, and scrapping on time, solder paste and flux remain consistent allies instead of hidden risks. Following FEFO and clear ready/not-ready rules keeps print and wave processes stable, predictable, and defect-free.