Mixed Model Production in PCB Manufacturing

Mixed model production is the practice of running many different products through the same line in short succession, and a board shop is a natural example of it. Orders differ by layer count, material, finish, thickness and size, and they arrive in quantities that rarely suit a single long run. The economics of the mixed model depend almost entirely on changeover time: when a changeover is short, small lots are affordable, and when it is long, the schedule is forced into batches that make everything slower.

Why the Mix Forces the Issue

A shop could improve its efficiency by running one product for a week and then the next, and it would deliver late to everybody. The alternative, running small lots of each product in turn, costs more setup time but delivers on time and keeps the work in progress low. The choice is therefore not between efficiency and flexibility in the abstract, but between two different cost structures that can both be calculated.

Customer behaviour pushes in the same direction. Order quantities have generally fallen and delivery expectations have shortened, and a shop that cannot run a ten panel order without disrupting a thousand panel order will find its small orders unprofitable and its large orders late. Flexible manufacturing is the capability to serve both, and it is built from changeover reduction rather than from additional machines.

Changeover Time Is the Lever

Every changeover has internal and external parts. Internal work happens while the machine is stopped, such as changing a stencil or a program. External work can be done while the machine is still running, such as fetching the tooling, preparing the material and checking the next program. Moving work from internal to external is the cheapest improvement available and it usually cuts changeover time substantially without any capital spending.

The second lever is standardisation. Common tooling, common panel sizes, common program structures and a consistent method for setting up a machine all reduce the time and the variability of a changeover. Setup reduction is therefore partly a technical exercise and partly a discipline, and the discipline part is the one that has to be maintained after the improvement project ends.

Measuring Changeover

Changeover time should be measured from the last good part of the previous product to the first good part of the next, because that is the interval during which the machine produces nothing saleable. Measuring from the start of the physical change understates the loss and usually understates the improvement potential as well. The measurement should be taken consistently, by the operator, with the start and end points agreed in advance.

It is worth recording the reasons as well as the duration. Waiting for a program, waiting for material, adjusting a parameter that should have been known and searching for a tool all appear as changeover time, and each has a different remedy. A simple sheet with the duration and the main reason is enough to show where the time is going, and it takes less effort than a stopwatch study.

Scheduling a Mixed Model Line

Scheduling a mixed line starts from the constraint. The sequence of products should be arranged so that the changeovers at the constraint are minimised, which may mean grouping products by material or by finish, while other steps are allowed to change over more freely because they have spare capacity. Optimising every step equally produces a schedule that is optimal nowhere.

Sequencing also has to respect due dates. A rule that groups similar products is useful until it delays an order that is already tight, so the schedule usually mixes a small amount of deliberate disruption with the grouping. Publishing the sequence a few days ahead lets the areas prepare externally, which is where much of the changeover reduction comes from in practice.

Small Lots and Work in Progress

Small lots reduce work in progress and shorten the feedback loop on quality. A defect made in a ten panel lot is discovered before it has been repeated a hundred times, which is one of the strongest arguments for the mixed model approach in a shop that is trying to improve. The savings from the reduced scrap often exceed the extra setup cost that the small lot creates.

The counter argument is that small lots consume capacity through changeovers. This is true and it is measurable, which is why the decision should be made from the changeover time rather than from an opinion. When the changeover is thirty minutes, a small lot is cheap; when it is four hours, the same lot is expensive, and the priority is to reduce the changeover rather than to argue about lot sizes.

Building the Capability

Flexible manufacturing is built from a series of small improvements rather than from a single project. Quick release tooling, pre-staged material, standardised programs, a setup checklist and a defined method for the first article check each remove a few minutes, and together they change the economics of the whole schedule. None of them requires new equipment.

The capability should also be maintained deliberately. Changeover time tends to creep back up as new products are introduced and standards erode, so the measurement should be continued after the improvement and reviewed with the process flow whenever a new product family is added. A shop that keeps its changeovers short can accept almost any order book, which is a commercial advantage as much as a production one.

Practical Rules

Measure changeover from last good part to first good part, move work from internal to external, and standardise the tooling and the method. Schedule around the constraint, publish the sequence in advance, and treat setup reduction as a continuing activity rather than a project.

Choose lot sizes from the measured changeover time and from the quality benefit of a shorter feedback loop, and record the reasons for long changeovers so that they can be attacked individually. Mixed model production is not a scheduling trick; it is the combination of a short changeover and a quality system that benefits from finding defects quickly.

Additional Considerations for This Build

Practical attention to production scheduling pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating production scheduling explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, setup reduction is the item that decides how the rest of the board is arranged. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

What is mixed model production? Running many different products through the same line in short succession, rather than batching each product into a long run.

What makes it affordable? Short changeovers. When setup takes minutes rather than hours, small lots cost little and the schedule becomes far more flexible.

How is changeover time measured? From the last good part of the previous product to the first good part of the next, including all waiting and adjustment.