1.2 Storage, Thawing, Handling, and Traceability

Solder paste is a time-sensitive chemical system, not a simpleshelf-stable commodity;commodity. Its performance degrades from the moment of manufacture due to flux reaction, metal oxidation, and solvent evaporation.

Strict adherence to the cold chain and handling protocols is not "administrative overhead"; it is athe fragileonly chemical system with an expiration date, and its performance window beginsway to closeguarantee the moment it leaves controlled storage. Every misstep in handling — from opening a cold jar to over-mixing — immediately impacts the paste's rheology, accelerates oxidation, and shrinks your printing process window.remains Disciplinedwithin the rheological window defined in Chapter 1.1. If you lose control overof the coldmaterial chainstate, isyou alose non-negotiablecontrol stepof for stabilizingthe print volume and protecting downstream yield.volume.

TheCold CriticalChain Path:Management Why(Storage Handling& RulesShelf Are Yield RulesLife)

Solder paste requiresmust extremebe discipline because it's a suspension of fine metal powder and reactive flux chemistrystored in a solvent.controlled Whenenvironment handlingto proceduresretard fail:the chemical reaction between the flux activators and the metal powder.

Storage Rules:

1. Condensation:Temperature: WaterMaintain contaminates0˚C the– flux,10˚C strictly.
   • Risk: > 10˚C accelerates flux activity, leading to spatter,viscosity micro-voids,rise (crust) and immediatereduced printwetting slumping.during reflow.
   • Risk: < 0˚C (Freezing) can cause flux separation if the formulation is not freeze-thaw stable. Check the Technical Data Sheet (TDS).
2. Oxidation:FIFO Mandate: Powder particles oxidize faster at room temperature, making it harder for the flux to clean them during reflow, resulting in poor wetting.
3. Rheology Damage: Over-mixing or excessive shear on the stencil breaks the solvent structure, leading to stringing and unpredictable volume release.

This section is the procedure for getting the paste from the fridge to the stencil without introducing yield loss.

Cold Chain Management: Storage and FIFO

The goal of cold storage is simple: slow down the chemical reactions (oxidation of the powder, degradation of the flux) and minimize solvent loss.

• Storage Temperature: Store sealed cartridges and jars at the vendor’s specified range, typically 0 – 10 °C (32 – 50 °F).
• Temperature Logging: Verify and log the 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):is non-negotiable.
  • Logic: EnforcePaste stricthas FIFOa basedfinite shelf life (typically 6 months). Using newer paste while older paste sits ensures you will eventually throw away expired, expensive material.
• Inventory Limit: Do not stockpile > 2 months of supply. Fresh paste always prints better than paste near its expiry date.

Thawing Protocol: The Thermodynamics of Condensation

The Golden Rule: Never open a jar or cartridge while it is cold.

When cold paste is exposed to warm ambient air, moisture instantly condenses on the surface (dew point).

• expirationEngineering dateConsequence: (orWater lotreacts code) to ensure paste isn't used afterwith the solventflux, causing "slump" (bridging) and fluxviolent systemsspattering have(solder degraded.balls) during reflow.

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.

Thawing Protocol: Avoiding the Condensation Trap

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

1. ClosedRemove Thaw:from Fridge: MovePlace the sealed container (jar or cartridge) fromin the fridgestaging toarea (20˚C – 25˚C).
2. Label: Mark the productioncontainer environmentwith (roomthe temperature:"Time Out of Refrigeration" timestamp.
3. Wait:20 –Allow 25natural ˚Cthermal equalization.
   • 500g Jars:) and allow it to warm upMinimum completely4 and unopenedhours.
   • The600g/1200g Why:Cartridges: OpeningMinimum a4 coldhours.
   • Syringes container(30cc): exposesMinimum the2 sub-zerohours.
4. Prohibited: pasteDo not use hot plates or heaters to warm,force-thaw.
   • Why: humidRapid air.heating Thecauses temperatureflux differenceseparation willand causelocalized moistureactivation.
to

Mixing condenseand onPreparation

Proper mixing restores the paste's surface—the dew point is met instantly. This water compromises the flux and will cause immediate printing defectsrheology (voiding,shear-thinning slump).

• Mandatorycapability) Soakafter Times:static Always adhere to the datasheet, but use these targets as a minimum:
  • Cartridge (300 – 600 g): 60 – 90 minutes
  • Jar (500 g): 2 – 4 hours

Process Note: Forced thawing (e.g., using a hot plate) is prohibited. The rapid heating can cause thermal shock and solvent separation, permanently damaging the paste's rheology.

Mixing, Shear, and Protecting Rheology

Once thawed, the paste must be homogenized, but not damaged.storage.

• InitialAutomated Mix:Softener (Preferred): Spin the jar for 1–3 minutes according to machine settings. This ensures consistent viscosity without introducing air bubbles.
• Manual Stirring (Fallback): Use a planetaryplastic mixerspatula. orStir gentle hand-rollinggently for a1–2 shortminutes duration,until typicallythe 30 – 60 seconds. The goaltexture is asmooth uniformand sheen,creamy.
  • Warning: Do not whipping it like cream.
  • The Danger of Shear: Over-mixing causes shear thinning—the internal polymer structure ofwhip the paste is broken down by friction.aggressively. This destroysintroduces theair paste’spockets, abilitywhich tobecome holdvoids aafter vertical print shape, leading to excessive slump and bridging on the PCB.reflow.
  • Temperature at Application: Paste temperature on the stencil should be kept stable (ideally 21-24 ˚C). Heat causes faster solvent evaporation, leading to the stringing and spikes you see on release.

Stencil ManagementLife and Open Time Policy

Stencil Life (Open Time) is the maximum time theOnce paste can be exposed to the airis on the stencilstencil, beforethe itsclock chemistrystarts degradesticking. Solvents evaporate, increasing viscosity and causing "dry prints" or incomplete aperture release.

Decision Logic: Paste Management on the Printer

• IF Production Stop < 60 minutes:
  • Then: Leave paste on stencil. Activate "Knead/Print" cycle every 10 minutes to keep it mobile.
• IF Production Stop > 1 hour:
  • Then: Remove paste from stencil. Place it in a "Used Paste" jar. Seal immediately. Clean stencil apertures.
• IF Paste has been on stencil > 8 hours (or TDS limit):
  • Then: Discard. Do not return to jar. The flux is exhausted.

The "Remix" Rule:

• Never put used paste back into a fresh jar. This contaminates the new paste with oxidized metal and dried flux.
• Reuse Strategy: You may mix up to 50% used paste with 50% fresh paste in a dedicated jar, provided the used paste is < 24 hours old. If in doubt, scrap it. The cost of one scrap board exceeds the cost of a scoop of paste.

Traceability Schema

You must be replaced.able to link a specific SPI defect trend to a specific batch of paste.

Required Data Linkage:

1. Paste Lot Number ↔ Printer Job ID: Record the Lot ID in the MES/Printer Setup Log before pouring.
2. Paste Expiry ↔ System Lockout: The MES should prevent the line from starting if the scanned paste barcode is expired.
3. Thaw Verification: Operator must input/scan the "Time Out" timestamp. If < 4 hours, process is blocked.

Traceability Scenario:

• Event: SPI shows sudden drift in volume/height across all pads.
• Check: Is this a new jar?
• Action: If yes, check Lot Number. If multiple lines fail with the same Lot, quarantine the Lot and contact the supplier.

Rejection Criteria: When to Scrap

Do not try to "save" bad paste. It is a sunk cost.

StepObservation	RuleDiagnosis	Best Practice for ControlAction
BeadCrust Sizeon Surface	MaintainOxidation/Drying adue small,to narrowlid beadleft (golf ball size) ahead of the squeegee.open.	Reason:Discard Jar. LargeDo "pancakes"not of paste expose maximum surface area tostir the air,crust acceleratingin; solventit loss.creates hard lumps that block apertures.
OpenFlux TimeSeparation	KnowClear theliquid Limitpool (typicallyon 4top –of 8grey hours, verify datasheet). Start a time-at-temperature log/timer when the paste hits the stencil.paste.	Control: Use timersDiscard. Indicates thermal shock or MESexpiry. loggingRemixing towill triggernot anrestore alarmproper when the limit is approached.chemistry.
End-of-Shift/PauseHard Lumps	Scoop-and-Scrap:"Rocks" Ifin the line pauses for more than 15-20 minutes, scoop the working bead off the stencil and cover it.paste.	Policy: NeverDiscard. returnWill pastedamage that has been exposed to the stencilsqueegee and airblock backstencils into the immediately.original, fresh jar.
PasteViscosity DispositionDrift	AtPaste therolls endpoorly (slides instead of therolling) shift,on 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.squeegee.	Risk:Discard. MixingSolvent exposedloss andis freshexcessive. pastePrinting contaminateswill thebe entire batch with oxidized powder.unstable.

QuickFinal Reference Card (Pin Near the Fridge)Checklist

StepParameter	ActionLimit / Rule	WhyControl Owner
Storage	0-10Fridge ˚C sealedTemp	Slows0˚C oxidation– &10˚C	Stores/Material solvent loss.Handler
Thaw Time	2 –≥ 4 Hours (jar) Sealed)Closed	PreventsLine deadly condensation.Operator
Mixing	30Shelf – 60s Gentle Roll/MixLife	Homogenizes< without6 breaking rheologyMonths (shearor thinning).per label)	MES / Incoming QC
Stencil Life	< 8 Hours (Continuous use)	Line Operator
Jar Open Time	TrackClose –lid Scrap/Replaceimmediately after scooping	OnceLine expired, print volume is unreliable.Operator
Bead SizeRemixing	KeepNever itmix Small/NarrowUsed into Fresh Jar	MinimizesLine air exposure/solvent loss on the stencil.Operator
End of Shift	SCARP Exposed PasteTraceability	PreventsScan contaminationLot ofID freshinto stock.Job Record	MES / Line Lead