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2.4 Board Handling & Line Control

Conveyors, buffers, SMEMA/IPC-Hermes handshakes, and warp/flatness compensation—everything between machines that keeps takt time steady.

Between the printer and reflow oven, the “invisible” parts of the line—conveyors, buffers, and communication protocols—decide whether production runs steady or stutters. Consistent conveyor setups and automatic width control keep panels centered and clamped without constant adjustment. Buffers act as shock absorbers, smoothing micro-stops so the slowest machine sets the pace without starving or blocking others. The long-standing SMEMA (Surface Mount Equipment Manufacturers Association) standard passes a simple “board ready / machine ready” signal between machines, ensuring smooth transfers, while the newer IPC-Hermes protocol adds intelligence—sending board size, ID, and recipe data ahead to cut changeover time and improve traceability. Board warp is best prevented in design, but in-line supports, vacuum tooling, and height mapping make even less-than-perfect panels behave. When these handling and control elements are standardized and tuned, boards flow in rhythm with takt time, and the rest of the process simply stays in sync.

2.4.1 Conveyors & width control (the quiet backbone)

  • Edge-belt conveyors with auto width keep boards centered and gaskets tight at the printer; teach three fiducials and let the lane hand you a board that’s already aligned.

  • Stop gate + lift pins/vac tooling at printers and mounters flatten the PCB where paste goes and where parts land. Your panel rails from design (2.5) give belts something sturdy to hold.

  • NG (No-Good) diverters route failed panels to a side buffer so the main lane never stalls.

House rule: pick one conveyor family per line for identical board stops, clamps, and width recipes—changeovers become button presses, not wrench time.

2.4.2 Handshakes: SMEMA vs IPC-Hermes (old switch vs smart chat)

  • SMEMA (classic): simple “board available / machine ready” dry contacts. Rock-solid, but blind—no board identity or recipe hints.

  • IPC-Hermes-9852 (modern): upstream sends board size, barcode/ID, and work step so downstream can pre-load the recipe and width, and attach scans to genealogy without extra readers. This is how your board-to-box story stays unbroken while lines change products.

If you can turn on Hermes, do it. Keep SMEMA as a safety net but let Hermes carry identity and width. Your buffers and NG gates get smarter overnight.

2.4.3 Buffers that smooth the beat

  • Inline accumulators before the constraint (slowest machine) absorb micro-stops upstream; a small one after it protects the rest of the line.

  • Loader/Unloader + Magazine buffer bookend the line for kit changes and WIP control.

  • Flip/Bypass modules: put them where double-sided flows want a clean “go around” path.

Sizing tip: estimate buffer slots = (largest single-machine micro-stop seconds ÷ line TAKT) × a safety factor. If you don’t know your micro-stops yet, start with 5–10 slots and adjust.

2.4.4 Warp & flatness compensation (make boards look flat)

Warped boards fake out paste and placement. Fight it in three layers:

A) Upstream prevention

  • Specify panel rails, tooling holes, and balanced stacks (Ch. 2.5) so fabrication and handling cause less bow.

B) At the station

  • Printer: map board support with pins/vac blocks right under big paste fields; if SPI shows regional lows, add pins before touching squeegee settings.

  • PnP: enable board height mapping (if available) or use dense supports; keep clamp force even so the head isn’t “chasing” Z.

  • Reflow: long, thin panels may need edge rails and center support fingers on the conveyor to stop sag through heat.

C) Process guardrails

  • Set a flatness spec at incoming (e.g., ≤0.75% of diagonal) and a rework path for outliers; don’t tune the whole line around one potato-chip panel.

2.4.5 Line-stop logic & NG flow (decide once, wire it everywhere)

  • Stop types: define soft stop (finish current board, hold handshakes) vs hard stop (halt upstream) and when each is used (jam, missing width change, recipe mismatch).

  • NG logic: AOI/SPI/Test flags push the panel ID via Hermes to NG buffers automatically; operators don’t chase paper tags. (This ties back to Chapter 4’s scanning.)

2.4.6 What planners & leads watch

  • Starvation / Blocked minutes at each machine (should fall after you add the right buffers).

  • Width-change time and recipe sync errors (Hermes should make these nearly vanish).

  • Board flatness rejects at printer clamp-down (trend vs lot/vendor—feed back to fab with your 2.5 rules).

2.4.7 Pocket checklists (post at the line PC)

Conveyors & buffers

  • Auto-width recipes saved per product; clamps/stops verified

  • Buffer before constraint; NG diverter tested; bypass path OK

Handshakes

  • SMEMA I/O healthy; Hermes IDs seen end-to-end (printer → mounters → AOI/test)

Flatness

  • Support pins/vac blocks placed under paste and BGA/QFN zones

  • Incoming warp screen active; outliers quarantined or re-fixtured



Bottom line: standardize conveyors and width recipes, let Hermes carry board identity, buffer the constraint, and flatten the PCB where it matters. Do that, and boards glide from printer to oven with no waiting, no surprises—and prints and placements look the same at 08:00 and 18:00.

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