FPC Design Parameters For Appliance Electronics

An appliance is a difficult environment for an interconnect. The inside of a washing machine, a refrigerator or a kitchen appliance is a three dimensional space occupied by motors, heaters, sensors and a control board, and the connections have to run between them without taking up the volume that a wire harness would need. A flexible printed circuit solves that problem, which is why so many appliances now use one.

This article covers the parameters that decide whether a particular FPC will work in an appliance, and how to read a supplier’s capability data against the demands of the product.

The list is short, but the parameters interact: a material chosen for high temperature also affects the bend life, and a thin construction that flexes well may not carry the current the heater needs.

Base Material And Conductor Type

The base material is usually polyimide or polyester. Polyimide is the more capable of the two, with a higher temperature rating and better dimensional stability, and it is the normal choice for appliances that generate heat or that will be reflowed during assembly. Polyester is cheaper and adequate for low temperature, static applications. The glass transition of the material sets the practical limit for soldering as well as for service, since a material that softens during assembly will not hold its dimensions during the rest of the process.

The conductor type matters for anything that bends. Rolled annealed copper has an elongated grain structure that tolerates repeated flexing, and electrodeposited copper, which is cheaper and adequate for a static installation, will crack in a dynamic application. The choice of base material and conductor together determines the flex life, and both should be stated rather than left to the supplier.

Geometry, Layer Count And Current

A single sided flexible circuit is the cheapest and the most flexible, and it is often enough for a sensor tail or a display connection. A double sided or multilayer construction is used where the routing does not fit on one layer, at the cost of stiffness and price. The minimum line width and spacing that the supplier can hold depends on the copper thickness, so the achievable density and the current to be carried have to be considered together rather than separately.

Where a circuit has to carry power to a motor or a heater, the trace has to be sized for the current with the same care as on a rigid board, and the thermal situation is less favourable because the material is thin and there is no plane to spread the heat. Sizing a trace for the current it carries is the first step, and the result may push the design toward a heavier copper weight and a coarser pitch.

Dynamic Flex Life And Temperature Range

Dynamic flex life is the number of bend cycles a circuit survives before a conductor breaks, and it is quoted under specified conditions of bend radius, angle and speed. For a moving part, an appliance design typically targets a hundred thousand cycles or more, and the number is meaningless without the conditions attached to it. A circuit that reaches that figure at a generous radius may fail in a few thousand cycles at a tight one.

The temperature range is quoted as a service range, and it is usually from minus forty to plus one hundred and five degrees, with kitchen appliances seeing higher local temperatures near heaters. The two parameters are linked, because a hot environment accelerates the ageing of the adhesive and the coverlay and reduces the flex life. Where both a wide temperature range and a high cycle count are required, the material and the geometry have to be chosen for them together.

Impedance Control On A Flexible Circuit

Where a flexible circuit carries a sensing signal or a fast digital link, the impedance of the traces becomes a design parameter, and it can be controlled on a flexible material in the same way as on a rigid board. The dielectric constant of polyimide is higher than that of FR-4, and the coverlay adds a layer above the trace, so the geometry that produces a given impedance is different and has to be calculated rather than copied from a rigid design.

The practical complication is that the geometry changes when the circuit bends, and so does the impedance, slightly. For most sensing applications that is irrelevant; for a high speed link it is worth understanding, and the design guidelines that apply to the whole board apply with the additional constraint of a compliant substrate.

Reliability Evidence And Verification

A supplier’s claim about flex life is worth as much as the test data behind it. The useful evidence is a bend test result on a similar construction, from the same material and with the same layer count, rather than a general statement that the material is flexible. Temperature and humidity cycling data covers the other half of the service conditions, and it is what reveals a coverlay or adhesive that will not survive.

It is worth confirming the quality characteristics that will be recorded for the finished part as well. Line width and spacing, copper thickness, coverlay registration and the result of the electrical test are the parameters that matter, and knowing they will be measured is what makes a specification meaningful rather than aspirational.

Stiffeners, Adhesives And Connectors

A flexible circuit is not uniformly flexible, and the design has to say where it should bend and where it should not. Stiffeners of polyimide or FR-4 are laminated behind connector areas and under components to give local rigidity, and they are what allow a zero insertion force connector to be plugged in without the substrate deforming. The adhesive system that bonds the coverlay and the stiffeners also has to survive the assembly temperature, since a bond that softens in the oven will let the layers shift.

The connector end deserves the same attention as the conductors. The termination method, the pad geometry at the tail and the strain relief behind it decide whether the part survives being plugged and unplugged repeatedly, which is often the most demanding mechanical event in the life of the product.

Choosing The Right Solution

The decision usually comes down to matching the material and the construction to the mechanical demand. A static connection inside a control box can be a single sided circuit on the cheapest material, with fine lines and a simple connector. A tail that flexes every time a door closes needs rolled copper, a polyimide base, a generous bend radius and a construction designed for the cycle count. The same product may contain both.

gopcb builds flexible and rigid flex circuits for appliance and industrial customers, and will advise on the construction that meets a given bend life rather than specifying the most capable material by default. Where a design is close to a limit, the useful conversation is about the bend radius and the number of cycles, because those two figures decide most of the cost.

FAQ

Is polyimide always necessary? No. Polyester is adequate for a static, low temperature application, and it costs less. Polyimide is used where the temperature or the flexing demands it.

What flex life should be specified? It depends on the number of cycles the part will see in service. The figure is only meaningful when the bend radius and the test conditions are stated with it.

Can a flexible circuit control impedance? Yes, and the calculation is similar to a rigid board, but the dielectric constant and the coverlay change the geometry required.