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2.2 Storage, Thawing & Handling

Solder paste is not a simple commodity; it is a fragile chemical system with an expiration date, and its performance window begins to close 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. Disciplined control over the cold chain is a non-negotiable step for stabilizing print volume and protecting downstream yield.

2.2.1 The Critical Path: Why Handling Rules Are Yield Rules

Solder paste requires extreme discipline because 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 harder 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 unpredictable volume release.

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

2.2.2 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): Enforce strict FIFO based on the expiration date (or lot code) to ensure paste isn't used after the solvent and flux 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.

2.2.3 Thawing Protocol: Avoiding the Condensation Trap

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

  1. Closed Thaw: Move the sealed container (jar or cartridge) from the fridge to the production environment (room temperature: 20 – 25 ˚C) and allow it to warm up completely and unopened.
  2. The Why: Opening a cold container exposes the sub-zero paste to warm, humid air. The temperature difference will cause moisture to condense on the paste's surface—the dew point is met instantly. This water compromises the flux and will cause immediate printing defects (voiding, slump).
  3. Mandatory Soak Times: 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.

2.2.4 Mixing, Shear, and Protecting Rheology

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

  • Initial Mix: Use a planetary mixer or gentle hand-rolling for a short duration, typically 30 – 60 seconds. The goal is a uniform sheen, not whipping it like cream.
  • The Danger of Shear: Over-mixing causes shear thinning—the internal polymer structure of the paste is broken down by friction. This destroys the paste’s ability to hold a vertical print shape, leading to excessive slump and bridging on the PCB.
  • 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.

2.2.5 Stencil Management and Open Time Policy

Stencil Life (Open Time) is the maximum time the paste can be exposed to the air on the stencil before its chemistry degrades and it must be replaced.

Step

Rule

Best Practice for Control

Bead Size

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

Reason: Large "pancakes" of paste expose maximum surface area to the air, accelerating solvent loss.

Open Time

Know the Limit (typically 4 – 8 hours, verify datasheet). Start a time-at-temperature log/timer when the paste hits the stencil.

Control: Use timers or MES logging to trigger an alarm when the limit is approached.

End-of-Shift/Pause

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

Policy: Never return paste that has been exposed to the stencil and air back into the original, fresh jar.

Paste Disposition

At the end 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: Mixing exposed and fresh paste contaminates the entire batch with oxidized powder.

Quick Reference Card (Pin Near the Fridge)

Step

Action

Why

Storage

0-10 ˚C sealed

Slows oxidation & solvent loss.

Thaw Time

2 – 4 Hours (jar) Closed

Prevents deadly condensation.

Mixing

30 – 60s Gentle Roll/Mix

Homogenizes without breaking rheology (shear thinning).

Open Time

Track – Scrap/Replace

Once expired, print volume is unreliable.

Bead Size

Keep it Small/Narrow

Minimizes air exposure/solvent loss on the stencil.

End of Shift

SCARP Exposed Paste

Prevents contamination of fresh stock.


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