Why Rated Speed Misleads Capacity Plans
Machine vendors publish theoretical placement rates, but real throughput depends on board layout, component mix, feeder positions, and board handling. In high-mix work, the sum of setup and idle time often exceeds run time. Plan capacity from measured data: log actual board time, changeover time, first article time, and downtime by shift for a few weeks.
Then calculate available hours as shift hours minus preventive maintenance, breaks, and expected changeover. The most common planning error is multiplying rated speed by calendar hours, which produces a plan the floor can never meet and trains planners to ignore it.
Changeover Is the Real Constraint
Each job change involves loading a new program, swapping stencils and printing parameters, setting up feeders for unique part numbers, and verifying the first board. The number of unique parts on a BOM is a better predictor of setup time than the number of placements. Shops reduce changeover by pre-staging kits and feeders offline, keeping common passives permanently loaded on shared feeder carts, and sequencing jobs by shared parts or similar board size.
Sequencing by due date alone causes extra setup, while sequencing by setup similarity alone causes late orders. The schedule should balance both, and a visible changeover time standard helps planners see the cost of moving a job.
- Pre-stage feeder carts and kits for the next job while the current one runs
- Keep high-use passives on semi-permanent feeders
- Group jobs sharing stencil thickness or many common parts
- Track actual changeover time against a standard
Clear to Build Gates and Material Shortages
A schedule is fiction if jobs are released without complete kits. Many EMS delays come from starting jobs short a few parts, then pausing the line or running partial builds that need a second pass through reflow. Set a clear to build gate: no job enters the SMT schedule until all components are in house or an approved plan covers the shortage, such as a first side only run.
Moisture sensitive devices add another constraint, since parts exposed past their floor life need baking before placement. The ERP should report shortages by job and due date so planners resolve them days before the slot, not at the line.
Try it here, free
SMT Setup Cost and Minimum Batch Size Calculator
Find setup cost per board for any batch size, and the minimum batch needed to keep setup under a target cost per board.
Downstream Bottlenecks: Reflow, AOI, Test and Rework
The SMT line is not always the bottleneck. Double-sided boards need two passes, selective solder and hand assembly may be limited by a few skilled operators, and test fixtures or flying probe time can constrain flow. X-ray inspection of BGAs is slow and is often a hidden bottleneck for complex boards.
Look at queue lengths between steps rather than only machine utilization. If work in process piles up before test, speeding up placement only grows the pile. Finite scheduling that models multiple constrained resources, not just the pick and place machine, gives realistic ship dates.
Where ERP Scheduling and AI Help, and Their Limits
ERPs such as SyteLine, Epicor, and others offer finite scheduling or APS modules, but they depend on accurate routings, setup times, and shortage data. If those inputs are wrong, the output will be too. AI can help planners by answering questions like which jobs due next week are short material, or where have we missed ship dates because of changeover, drawing on live ERP data.
Netray builds this as a read-only layer over your ERP. It will not replace an APS engine or fix inaccurate routing times, so measure and correct your standards first.
Frequently asked questions
How do I calculate SMT line capacity in a high-mix shop?
Start with scheduled hours, subtract maintenance, breaks, and measured changeover and first article time, then apply measured board time per job. Use actual data from several weeks rather than rated machine speed. The result is the available run time, which you can load with jobs based on real cycle times.
Should I sequence SMT jobs by due date or by setup similarity?
Neither alone. Due date only sequencing causes excessive changeovers, while grouping purely by setup similarity makes orders late. Most shops use due date priority with limited grouping of jobs sharing stencils or many common parts, and they track the cost of expediting exceptions.
What does clear to build mean in EMS?
A job is clear to build when all required components, boards, stencils, and tooling are available or covered by an approved plan before it enters the schedule. It prevents starting jobs that stall on shortages. Many shops enforce it as a formal gate in the ERP before releasing the work order to the floor.
If your schedule keeps slipping because of shortages and changeovers, Netray can help you query your ERP for the pattern and show where the plan and the floor diverge.