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1.2 Storage, Thawing & Handling

Temperature control, mixing, open time, and how to avoid paste death by shear or oxidation.

Solder paste is a temperature-sensitive, chemically active material, and the way it’s stored and handled can decide whether printing runs smoothly or turns into a shift full of reprints and cleaning stops. Every step—cold storage, controlled thawing, gentle mixing, and limiting its exposure on the stencil—affects viscosity, wetting, and consistency. Poor handling accelerates oxidation, drives off solvents, and breaks down the flux, shrinking your process window before parts even see reflow. A disciplined cold-chain and on-line routine keeps the paste within its designed chemistry, giving predictable print volumes, stable SPI readings, and fewer downstream defects.

1.2.1 The one-minute story (why care?)

Solder paste is alive—well, chemically. Treat it gently and it prints like butter for hours. Abuse it (wrong temp, over-mixing, leave it breathing on a hot stencil) and it turns stringy, slumps, oxidizes, and stops wetting. This section is the “keep it happy” routine your line can follow without thinking.



1.2.2 Fridge to stencil: the smooth path

Storage (before use)

  • Keep sealed jars/cartridges refrigerated at the vendor’s range (commonly 0–10 °C).

  • FIFO by lot/expiry; log temps on receipt (no mystery boxes).

  • Don’t freeze unless the datasheet explicitly says so.

Thawing (so you don’t make soup)

  • Move closed containers to the line in advance; let them warm to room temp (20–25 °C) before you open.

  • Typical soak times (guidance—use your vendor’s sheet):

    • Cartridge (300–600 g): 45–90 min

    • Jar (500 g): 2–4 h

  • Why closed? Condensation will form if you open cold paste; water + flux = paste ruin.

First mix (homogenize, don’t whip)

  • Gently hand-roll or use a planetary mixer for ~30–60 s. Goal: uniform sheen, no air bubbles.

  • Over-mixing = excess shear → heat + broken rheology.



1.2.3 Printer-side habits that make paste last

  • Bead size: keep a golf-ball–to–thumb bead ahead of the squeegee, not a pancake across the stencil. Top up little and often.

  • Open time: know your paste’s stencil life (often 4–8 h). If you pause >15–20 min, scoop the bead off, park it covered, and run an understencil clean before restarting.

  • Environment: 21–24 °C, 40–60 % RH is a friendly pocket. Hot + dry rooms evaporate solvents and shrink the window.

  • Don’t mix worlds: never return stencil-exposed paste to the original jar. Keep a small “stencil-use only” cup if your policy allows reuse the same shift; otherwise scrap to metal-waste.

  • End of shift: remove paste from stencil, wipe, run a final dry/vac clean. Reseal cartridges; jars go back to the fridge only if they’re within open-time and still clean.



1.2.4 Shear & oxidation: recognizing early warning signs

Symptom on printer

Likely cause

First fix

Stringing/tails on release

Over-mixed, or bead too warm/dry

Reduce bead size; add fresh paste; shorten mix; verify room temp/RH

Slump/bridging at fine pitch

Paste too hot or solvent loss

Cool the room; shorten pauses; refresh bead; check powder type vs aperture ratios

Grainy edges / poor fill

Oxidized powder, exhausted flux

Replace bead; check jar age and storage log

Wetting looks lazy downstream

Flux activation tired (age/over-shear)

Switch to a fresh jar; consider N₂ in reflow for marginal combos (see 9.3)



1.2.5 Quick numbers (pin near the fridge)

Step

Typical target

Why it matters

Storage

0–10 °C, sealed

Slows oxidation/solvent loss

Thaw

Room temp, closed; 45–90 min (cartridge), 2–4 h (jar)

Avoids condensation; restores print rheology

Mix

30–60 s gentle roll/planetary

Homogenize without whipping air

Stencil life

4–8 h (product-specific)

Plan top-ups & pauses accordingly

Bead size

Narrow ribbon, ~10–15 mm wide

Keeps shear/heat down; improves release

(Always defer to your paste datasheet; these are sane starting points.)



1.2.6 Handling rules you can enforce with systems

  • Timers: start “time at temperature” when paste leaves the fridge; start stencil open-time when it hits steel. Use simple MES prompts instead of sticky notes.

  • Barcode everything: paste lot → printer → work order. If a lot expires, the printer login should block use.

  • Two-bin policy: an in-use container at the printer and a backup staged/soaking; no mid-run fridge sprints.

  • Training snippet: operators should recognize the four symptoms in 7.2.4 and know the first fix without calling engineering.



1.2.7 Special cases

  • Fine powders (Type 4/5): smaller particles = faster oxidation. Keep rooms a touch cooler, shorten pauses, and consider nitrogen in reflow if wetting is marginal.

  • Low-temp (Bi-based) pastes: more sensitive to over-mix/heat; be strict on soak and open-time.

  • Water-soluble flux: plan real post-reflow cleaning; don’t stretch open-time—activators run hot and expire faster.



1.2.8 Pocket checklists

At the fridge

  • FIFO by lot/expiry · [ ] Temp log OK · [ ] Container intact/sealed

Thaw & mix

  • Warmed closed to room temp · [ ] Gentle 30–60 s mix · [ ] No condensation

On the printer

  • Small bead; top-up often · [ ] Track open-time · [ ] Pause routine (scoop + clean) posted

End of shift

  • Paste off stencil · [ ] Stencil wiped/cleaned · [ ] In-use container dispositioned (reuse same shift only or scrap per policy)



Bottom line: cold-chain right, thaw closed, mix gently, keep a small bead, and time your pauses. Do those five things and you’ll stop most paste-related print drama before it starts—and your SPI charts will look satisfyingly boring.


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