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2.4 Board Handling & Line Control

The flow of panels between SMT stations — handled by conveyors, buffers, and communication links — is critical for achieving stable throughput. Line control ensures that boards are presented correctly to the machinery, flattened for precision placement, and moved efficiently without causing jams or starvation. The shift from blind communication (SMEMA) to intelligent networking (IPC-Hermes) provides the foundation for full line synchronization, automated changeovers, and complete product traceability.

2.4.1 Conveyors and Physical Constraints

Conveyors form the physical backbone of the line. Their consistent operation is necessary for stable machine alignment.

  • Width Control and Centering: Automated edge-belt conveyors with auto-width adjustment are essential. They ensure boards are always centered and presented to the printer's tooling plate or mounter's clamp mechanism at a known, consistent position.
  • Warp Mitigation (The Flatness Problem): Boards must be held flat at the point of process. Stop gates with integrated lift pins or vacuum blocks are non-negotiable at the printer (for paste flatness) and the pick-and-place machine (for placement accuracy).
  • Standardization: House Rule: Standardize the conveyor family across the entire SMT line. This ensures identical board stops, clamping force, and width recipes between machines, making product changeovers repeatable and rapid.
  • NG Diverters: Dedicated NG (No-Good) diverters must be installed (typically after SPI and AOI) to automatically route defective panels to a side buffer without halting the main production flow.

2.4.2 Handshake Protocols: Blind Transfer vs. Smart Genealogy

The communication protocol dictates the level of line automation and data integrity.

Protocol

Function

Data Transfer (The "Handshake")

Managerial/Traceability Value

SMEMA (IPC-SMEMA-10)

Classic/Baseline. Simple electrical signals.

Blind. Sends only "Board Available" or "Machine Ready" via dry contact.

Zero. Requires manual barcode scanning at each station to break board identity.

IPC-Hermes-9852

Modern/Smart. TCP/IP based.

Intelligent. Passes Board ID, size, product type, and recipe name downstream.

High. Enables recipe pre-loading, automated width changes, and unbroken board genealogy.

Action: If the equipment supports it, Hermes must be enabled. It cuts down changeover time (OpEx) by pre-loading parameters and is the foundation for full traceability without relying on repetitive barcode scans.

2.4.3 Buffers and Line Synchronization

Buffers are deliberately sized inventory zones that absorb micro-stops and decouple line speed variations, protecting the overall Takt.

  • Constraint Buffering: The most critical buffer must be placed immediately before the constraint (the slowest machine/bottleneck). This ensures the bottleneck is never starved due to micro-stops upstream.
  • Decoupling: A smaller buffer placed after the constraint protects the downstream reflow oven from micro-stops at the bottleneck, maintaining a consistent thermal profile pace.
  • Sizing Principle: Buffer slots should be sized based on the longest average micro-stop time of the slowest machine divided by the line Takt, plus a safety margin. Short boards or complex changeovers often require larger buffers.

2.4.4 Warp, Flatness, and Process Guardrails

Warped boards are a primary source of printing and placement error. Prevention and control are essential.

  1. Incoming Material Spec: Boards must meet a strict flatness tolerance at incoming quality control (IQC), typically ≤ 0.75% of the panel diagonal. Panels exceeding this limit should be quarantined.
  2. Station Compensation: Machines must utilize height mapping and dense support to physically constrain the board. Reflow ovens handling long or thin panels should use center support fingers to prevent sag that causes solder movement (opens) during the peak thermal zone.
  3. Line-Stop Logic: Define clear criteria for Soft Stops (finish current board, hold handshakes) and Hard Stops (emergency halt). NG logic must be wired to the main communication system so that SPI or AOI alarms automatically trigger the NG diverter, preventing further processing of a faulty unit.

Final Checklist: Operational Flow

Area

Uptime Goal

Managerial Focus

Conveyor

Auto-width recipes verified; support pins/vac blocks placed under all BGA/fine-pitch areas.

Reduce time spent on manual width changes.

Handshake

Hermes enabled and verified end-to-end (printer to AOI/Test).

Eliminate recipe mismatch errors and auto-log genealogy.

Buffers

Buffer placed before the Constraint; Starvation/Blocked Minutes dashboard metric trend is zero.

Protect Takt Time and maximize OEE.

Flatness

Incoming inspection active; Warp Screen confirms adequate support before process start.

Prevent yield loss due to random panel distortion.


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