1.2 Storage, Thawing, Handling, and Traceability

Solder paste is a time-sensitive chemical system, not a shelf-stable 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 the only way to guarantee the printing process remains within the rheological window defined in Chapter 1.1. If you lose control of the material state, you lose control of the print volume.

Cold Chain Management (Storage & Shelf Life)

Solder paste must be stored in a controlled environment to retard the chemical reaction between the flux activators and the metal powder.

Storage Rules:

1. Temperature: Maintain 0˚C – 10˚C strictly.
   • Risk: > 10˚C accelerates flux activity, leading to viscosity rise (crust) and reduced wetting 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. FIFO Mandate: First-In, First-Out is non-negotiable.
   • Logic: Paste has a finite shelf life (typically 6 months). Using newer paste while older paste sits ensures you will eventually throw away expired, expensive material.
3. 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).

• Engineering Consequence: Water reacts with the flux, causing "slump" (bridging) and violent spattering (solder balls) during reflow.

Thawing Procedure:

1. Remove from Fridge: Place the sealed container in the staging area (20˚C – 25˚C).
2. Label: Mark the container with the "Time Out of Refrigeration" timestamp.
3. Wait: Allow natural thermal equalization.
   • 500g Jars: Minimum 4 hours.
   • 600g/1200g Cartridges: Minimum 4 hours.
   • Syringes (30cc): Minimum 2 hours.
4. Prohibited: Do not use hot plates or heaters to force-thaw.
   • Why: Rapid heating causes flux separation and localized activation.

Mixing and Preparation

Proper mixing restores the paste's rheology (shear-thinning capability) after static storage.

• Automated 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 plastic spatula. Stir gently for 1–2 minutes until the texture is smooth and creamy.
  • Warning: Do not whip the paste aggressively. This introduces air pockets, which become voids after reflow.

Stencil Life and Open Time

Once paste is on the stencil, the clock starts ticking. 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 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.

Observation	Diagnosis	Action
Crust on Surface	Oxidation/Drying due to lid left open.	Discard Jar. Do not stir the crust in; it creates hard lumps that block apertures.
Flux Separation	Clear liquid pool on top of grey paste.	Discard. Indicates thermal shock or expiry. Remixing will not restore proper chemistry.
Hard Lumps	"Rocks" in the paste.	Discard. Will damage squeegee and block stencils immediately.
Viscosity Drift	Paste rolls poorly (slides instead of rolling) on squeegee.	Discard. Solvent loss is excessive. Printing will be unstable.

Final Checklist

Parameter	Limit / Rule	Control Owner
Fridge Temp	0˚C – 10˚C	Stores/Material Handler
Thaw Time	≥ 4 Hours (Sealed)	Line Operator
Shelf Life	< 6 Months (or per label)	MES / Incoming QC
Stencil Life	< 8 Hours (Continuous use)	Line Operator
Jar Open Time	Close lid immediately after scooping	Line Operator
Remixing	Never mix Used into Fresh Jar	Line Operator
Traceability	Scan Lot ID into Job Record	MES / Line Lead