6.6 Strict MSD handling & baking

Moisture Sensitive Devices (MSDs) present a critical process risk. The concern is moisture accumulating microscopically within the plastic component encapsulant. During the intense heat of SMT reflow soldering (240°C+), this trapped moisture vaporizes into steam, expanding to crack the package (the “popcorn” effect) or delaminate the silicon die. This structural failure is often internal and invisible to Automated Optical Inspection (AOI), allowing it to pass initial electrical testing but fail later in the field.

Treat MSD control as a strict Time-Based Ledger. Exposure to ambient air reduces the component’s calculated Floor Life. Once that exposure time reaches its limit, the part must be baked or scrapped.

The MSL hierarchy

Determine MSL ratings primarily based on manufacturer specifications (often per IPC/JEDEC J-STD-033), not by visual estimation.

• MSL 1: Unlimited Floor Life (Safe for extended exposure).
• MSL 2 – 2a: 1 year down to 4 weeks of exposure.
• MSL 3: 168 Hours (7 Days). This is common for many complex ICs and requires careful tracking.
• MSL 4 – 5a: 72 down to 24 Hours. Highly sensitive; requires strict clock management.
• MSL 6: Mandatory bake required immediately before use. Zero floor life.

The floor life exposure clock: start, pause, reset

The ERP/WMS should systematically track the “Open Time” for each unique component reel to avoid relying on human memory.

• Opening the Barrier Bag (The Clock Starts):
  • Inspect the Humidity Indicator Card (HIC) as soon as the Moisture Barrier Bag (MBB) is opened.
  • The Protocol: If the HIC indicates safe levels (critical spots remain Blue/Brown), start the digital Floor Life Clock.
  • The Protocol: If the HIC indicates excessive moisture (the relevant spot has turned Pink), hold the material. Move it to Quarantine for baking; do not issue it to the line.
• Dry Cabinet Storage (The Clock Pauses):
  • Storing parts in an active Dry Cabinet (< 5% RH) pauses the exposure clock. It does not reset the clock to zero unless the part is stored there for an extended duration (refer to J-STD-033 for calculating recovery time).
• Vacuum Re-Sealing (The Clock Stops):
  • Vacuum sealing the part in a fresh MBB with new desiccant and a new HIC stops the clock.
  • Pro-Tip: Do not reuse old desiccant pouches. Once exposed, they become saturated and ineffective.

Baking logic: the reset

Baking imposes thermal stress on components and accelerates lead oxidation, which can degrade solderability. Treat baking as a corrective action, not a default processing step.

• When to Bake:
  • If the Floor Life is exceeded (e.g. an MSL 3 part exposed for > 168 hours).
  • If the HIC explicitly indicates dangerous moisture levels upon opening.
  • If handling traceability is lost, making the remaining floor life unknown.
• Baking Constraints:
  • Tape & Reel: Use a low-temperature bake (typically 40°C for an extended duration). High-temperature baking (e.g. 125°C) will melt standard plastic carrier tape.
  • JEDEC Tray / Bulk: High-temperature baking (e.g. 125°C) is permitted ONLY if the physical tray is explicitly rated for high temperatures (verify the max temp stamped on the tray).
  • The Cycle Limit: Baking a component multiple times is generally discouraged due to oxidation risks. Track bake cycles carefully.

Handling partially opened reels & trays

Returning partially used SMT reels to the warehouse requires strict handling to maintain control.

• The Time Guideline: Avoid leaving exposed reels sitting unattended on kitting tables. Define a maximum allowable window (e.g. 15–30 minutes) from “Physical Line Return” to “Vacuum Sealer.”
• Digital Labeling: The freshly sealed bag must carry a new label stating:
  • The MSL Level.
  • The Remaining Floor Life (e.g. “Remaining Floor Life: 45 Hours”).
• Vacuum Seal Quality: Ensure the MBB is tightly vacuumed. A loose or “soft” bag suggests a leak.
  • The Protocol: If an MBB loses its vacuum while in storage, treat the component as exposed and calculate the risk accordingly.

Final Checkout: Strict MSD handling & baking

Trigger Condition	Required Protocol	Typical Owner
HIC Check	If the critical spot is Pink, block the material in the ERP and arrange for baking.	Receiving / Kitting
Missing Digital Log	If “Time on Shelf” is unknown, assume exposure limits are breached. Bake before use.	Warehouse Leadership
Physical Bake Temp	Ensure tape/reel components are baked at low temps (e.g. 40°C) to prevent melting.	Process Engineering
The Re-Seal	Always use new desiccant and a new HIC when resealing MBBs.	Kitting / Warehouse Staff
Floor Life Logic	Ensure remaining exposure time safely exceeds the planned SMT production cycle time.	Material Planner / Kitting