6.1 Changeover Reduction (SMED)

In high-mix electronics manufacturing, a common challenge to Overall Equipment Effectiveness (OEE) is the idle time between production runs. An SMT line is most valuable when it is actively building. Single Minute Exchange of Die (SMED) is the operational practice of reducing changeover time, ideally targeting single digits (<10 minutes). Separating “Internal” from “External” setup activities helps transform the changeover into a streamlined, coordinated process.

The SMED Philosophy: Internal vs. External

To support rapid changeovers, it is helpful to categorize and sequence tasks carefully.

• External Setup (OED): Activities performed while the machine is still running the previous product.
  • Goal: Aim to shift approximately 80% of preparation work to this phase.
  • Examples: Mounting reels to offline feeder carts, removing solder paste from refrigeration to warm up, retrieving stencils, verifying BOM data.
• Internal Setup (IED): Activities performed while the machine is stopped.
  • Goal: Keep this duration as brief as safely possible.
  • Examples: Unclamping the stencil, swapping the feeder cart, adjusting conveyor width.
• The Principle: External tasks are ideally not performed while the machine is stopped. For instance, searching for a stencil key or a missing reel during downtime extends the internal setup unnecessarily.

External Preparation Methods

Effective preparation is usually completed and validated before the current run finishes.

• Feeder Kitting: Components are loaded onto Mobile Feeder Trolleys or Exchange Banks offline.
  • Validation: Try to scan and link all reels to the feeder slot index using an offline docking station. This helps eliminate the need to scan and verify during the active downtime window.
• Stencil & Paste:
  • The next stencil should be retrieved and visually inspected for damage prior to changeover (often 30 minutes in advance).
  • Solder paste is removed from the refrigerator early enough (often 4 hours prior) to ensure it reaches ambient temperature naturally before the line stops.
• Conveyor Settings: The width of the next PCB is pre-measured. Manual conveyor spacers are set, or the digital width setting is recorded for quick entry.

Internal Execution (The “Pit Stop”)

When the last board of the previous product exits the reflow oven, the changeover begins. This process benefits significantly from a coordinated team approach, with technicians working concurrently where safe and practical.

1. Printer Station:
   • The used stencil and squeegees are removed promptly.
   • The new stencil is loaded, and fresh, pre-stirred paste is applied.
   • New support pins (magnetic or vacuum) are placed, often aided by a dedicated setup jig.
2. Mounter Station:
   • Bank Swapping: The entire feeder bank for the previous product is undocked, and the pre-loaded bank for the new product is docked in its place. Swapping individual feeders is typically avoided during this phase to save time.
   • Support: Board support pins are adjusted to the defined coordinate map to help prevent board flex.
3. Reflow Oven:
   • The new thermal recipe is loaded immediately. Modern ovens require time (often 5-15 minutes) to stabilize temperatures, making this the logical first step rather than the last.

Streamlining the Ramp-Up (First Article)

The changeover concludes when the first acceptable board is produced. The First Article Inspection (FAI) can become a bottleneck if not optimized.

• Electronic FAI: A dedicated First Article Scanner (e.g., a scanner with golden sample overlay comparison) is often used instead of manual caliper or microscope checks. This helps keep FAI completion efficient, often under 3 minutes.
• Setup Boards: A non-functional “Setup Board” can be run to verify print alignment and placement polarity before committing live, valuable components.
• Offline Corrections: It is best practice to correct program coordinates in the offline CAD data rather than “teaching” them directly on the machine during production. This ensures the master machine configuration remains consistent for future runs.

SMED Metric Guidelines

Metric / Action	Target / Standard Practice
Changeover Target	Aim for < 10 Minutes (Last Good Board to First Good Board)
External Prep	Complete before line stop
Feeder Strategy	Mobile Cart Exchange (minimize single swaps)
Validation	Perform Offline Scanning
Oven Profile	Load First (to allow thermal stabilization)
Paste Condition	Verify Ambient Temp naturally reached
Personnel	Consider 2 Technicians (concurrent method)
Documentation	Utilize a Digital Checklist