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1.2 Storage, Thawing & Handling

Solder paste is not a livingsimple materialcommodity; init theis factorya sense—sensitivefragile tochemical time,system temperature,with an expiration date, and handling. Itsits performance window begins to narrowclose the moment it leaves coldcontrolled storage,storage. and everyEvery misstep in thawing,handling mixing,— orfrom stencilopening managementa cold jar to over-mixing—immediately impacts the paste's rheology, accelerates itsoxidation, decline.and Whatshrinks seemsyour likeprinting process window. Disciplined control over the cold chain is a simplenon-negotiable consumablestep is actually a fragile chemical balance, where solvent retention, flux activity, and powder surface condition all decide whether prints release cleanly or collapse into defects. Controlling the cold chain and daily routines keeps the paste predictable,for stabilizing print qualityvolume and protecting downstream yield.

1.2.1 The one-minuteCritical storyPath: (whyWhy care?)Handling Rules Are Yield Rules

Solder paste isrequires alive—well,extreme chemically.discipline Treatbecause it's a suspension of fine metal powder and reactive flux chemistry in a solvent. When handling procedures fail:

  • Condensation: Water contaminates the flux, leading to spatter, micro-voids, and immediate print slumping.
  • Oxidation: Powder particles oxidize faster at room temperature, making it gentlyharder for the flux to clean them during reflow, resulting in poor wetting.
  • Rheology Damage: Over-mixing or excessive shear on the stencil breaks the solvent structure, leading to stringing and itunpredictable printsvolume likerelease.
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 “keepprocedure itfor happy”getting routinethe yourpaste linefrom canthe followfridge to the stencil without thinking.

introducing


yield


loss.

1.2.2 FridgeCold toChain stencil:Management: Storage and FIFO

The goal of cold storage is simple: slow down the smoothchemical pathreactions

Storage (beforeoxidation use)of the powder, degradation of the flux) and minimize solvent loss.

  • Storage Temperature:Keep Store sealed jars/cartridges refrigeratedand jars at the vendor’s rangespecified (commonlyrange, typically 0–0 – 10 °C) (32 – 50 °F).
  • FIFOTemperature Logging: byVerify lot/expiry;and log tempsthe temperature of the paste upon receipt. If the seal is broken or the paste is received warm, reject the batch immediately.
  • First-In, First-Out (FIFO): Enforce strict FIFO based on receiptthe expiration date (noor mysterylot boxes).
  • Don’code) to ensure paste isn't freezeused unlessafter the datasheetsolvent explicitlyand saysflux so.systems have degraded.

System Enforcement: The Manufacturing Execution System (MES) or inventory control system should be configured to block any work order from retrieving expired or non-compliant paste lots.

1.2.3 Thawing (soProtocol: youAvoiding don’tthe makeCondensation soup)Trap


The thawing process is the most common point of failure. Never open a cold jar.

    1. Closed Thaw: Move closedthe containerssealed container (jar or cartridge) from the fridge to the lineproduction environment (room temperature: 20 – 25 ˚C) and allow it to warm up incompletely advanceand unopened.
    2. The Why:; letOpening thema warmcold container exposes the sub-zero paste to roomwarm, temphumid (20–25air. °C)The temperature difference will cause moisture to condense on the paste's surface—the beforedew you openpoint is met instantly. This water compromises the flux and will cause immediate printing defects (voiding, slump).
    3. Mandatory Soak Times:Typical soakAlways timesadhere (guidance—to the datasheet, but use yourthese vendor’stargets sheet):as a minimum:
      • Cartridge (300–300 – 600 g): 45–60 – 90 minminutes
      • Jar (500 g): 2–2 – 4 hhours
    5. Why
    closed?

    Process CondensationNote: willForced formthawing if(e.g., youusing opena coldhot paste;plate) wateris +prohibited. fluxThe =rapid heating can cause thermal shock and solvent separation, permanently damaging the paste's rheology.

    1.2.4 Mixing, Shear, and Protecting Rheology


    Once thawed, the paste ruin.

must

Firstbe mixhomogenized, (homogenize,but don’tnot whip)damaged.

  • GentlyInitial hand-rollMix: or useUse a planetary mixer or gentle hand-rolling for ~30–a short duration, typically 30 – 60 s.seconds. Goal:The goal is a uniform sheen, nonot airwhipping bubbles.it like cream.
  • The Danger of Shear: Over-mixing = excesscauses shear thinning—the →internal heatpolymer + broken rheology.



1.2.3 Printer-side habits that make paste last

  • Bead size: keep a golf-ball–to–thumb bead aheadstructure of the squeegee,paste notis broken down by friction. This destroys the paste’s ability to hold a pancakevertical acrossprint shape, leading to excessive slump and bridging on the stencil. Top up little and often.PCB.
  • OpenTemperature time:at Application: knowPaste yourtemperature paste’s stencil life (often 4–8 h). If you pause >15–20 min, scoopon the beadstencil offshould be kept stable (ideally , park it covered, and run an understencil clean before restarting.
  • Environment: 21–21-24 °C,˚C). 40–60Heat %causes RHfaster issolvent aevaporation, friendly pocket. Hot + dry rooms evaporate solvents and shrink the window.
  • Don’t mix worlds: never return stencil-exposed pasteleading to the originalstringing jar. Keep a smalland “stencil-use only”spikes cupyou ifsee youron policyrelease.
allows

1.2.5 reuseStencil Management and Open Time Policy

Stencil Life (Open Time) is the samemaximum shift;time otherwise scrap to metal-waste.

  • End of shift: removethe paste fromcan stencil,be wipe, run a final dry/vac clean. Reseal cartridges; jars go backexposed to the fridgeair onlyon ifthe they’restencil withinbefore open-timeits chemistry degrades and stillit clean.must be replaced.



    • 1.2.4 Shear & oxidation: recognizing early warning signs

    Symptom on printerStep

    Likely causeRule

    FirstBest fixPractice for Control

    Stringing/tailsBead Size

    Maintain a small, narrow bead on(golf releaseball size) ahead of the squeegee.

    Over-mixed, or bead too warm/dry

    Reduce bead size; add freshReason: paste;Large shorten"pancakes" mix;of verifypaste roomexpose temp/RHmaximum surface area to the air, accelerating solvent loss.

    Slump/bridgingOpen Time

    Know the Limit at(typically fine4 pitch– 8 hours, verify datasheet). Start a time-at-temperature log/timer when the paste hits the stencil.

    Control:Paste tooUse hottimers or solventMES loss

    loggingCool to trigger an alarm when the room;limit shortenis pauses; refresh bead; check powder type vs aperture ratiosapproached.

    Grainy edges / poor fillEnd-of-Shift/Pause

    Scoop-and-Scrap:Oxidized powder,If exhaustedthe fluxline pauses for more than 15-20 minutes, scoop the working bead off the stencil and cover it.

    Policy:Replace bead;Never checkreturn jarpaste agethat has been exposed to the stencil and storageair logback into the original, fresh jar.

    WettingPaste looksDisposition

    At lazythe downstreamend of the shift, all stencil-exposed paste should be classified as waste (for metal recycling) or transferred to a separate, controlled "re-use" jar for limited same-day application per your quality policy.

    Risk:Flux activationMixing tiredexposed (age/over-shear)

    Switch to aand fresh jar;paste considercontaminates N₂the inentire reflowbatch forwith marginaloxidized combos (see 9.3)powder.



    1.2.5 Quick numbersReference Card (pinPin nearNear the fridge)Fridge)

    Step

    Typical targetAction

    Why it matters

    Storage

    0–0-10 °C,˚C sealed

    Slows oxidation/oxidation & solvent lossloss.

    Thaw Time

    2 – 4 Hours (jar) Closed

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

    Avoids condensation; restores print rheologycondensation.

    Mixing

    30 – 60s Gentle Roll/Mix

    30–60 s gentle roll/planetary

    HomogenizeHomogenizes without whippingbreaking airrheology (shear thinning).

    StencilOpen lifeTime

    Track – Scrap/Replace

    4–8Once hexpired, (product-specific)

    Planprint top-upsvolume &is pauses accordinglyunreliable.

    Bead sizeSize

    Keep it Small/Narrow

    NarrowMinimizes ribbon,air ~10–15exposure/solvent mmloss wideon the stencil.

    End of Shift

    SCARP Exposed Paste

    KeepsPrevents shear/heatcontamination down;of improvesfresh releasestock.

    (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)



    Conclusion: Maintain disciplined storage, thawing, mixing, and stencil-use practices, reinforced by simple systems and operator training. Doing so preserves paste chemistry, minimizes print variation, and prevents costly interruptions on the line.


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