Laser Cutting and Drilling of Flexible Materials

Lasers cut and drill flexible materials without the mechanical force that a router or a punch applies, which is why they are used on thin films and on adhesives. The same beam that removes material also leaves a heat affected zone, and the width of that zone is the parameter that decides whether the result is acceptable.

Why Lasers Suit Flex

A flexible circuit is thin and unsupported, and a mechanical tool pushes on it. The laser cuts by removing material, so the film can be held flat by vacuum without being clamped, and the edge is formed without a burr that would have to be removed.

Lasers also cut patterns that a die cannot, which matters for prototype and for a shape that is not worth a tool. Our multilayer flex notes describe how that material is built up before it is cut.

Wavelength and Material Absorption

Each material absorbs a different part of the spectrum, so the choice of laser follows the choice of material. A wavelength that is absorbed at the surface removes a film cleanly, and one that passes through heats the layers underneath instead.

A polyimide film absorbs well in the ultraviolet, which is why that family is common for flex cutting and for drilling the small holes in a thin film. Our flex materials notes describe the films that are normally used.

The Heat Affected Zone

Around every cut there is a region where the material has been heated without being removed. In a polyimide that region is carbonised, and carbonised polyimide is conductive enough to matter on a fine pattern.

The zone is controlled by the pulse energy, the pulse length and the number of passes. A shorter pulse removes material before the heat can spread, so a process with many short passes produces a narrower zone than one with a single heavy pass. Our microvia notes describe the same control for the small holes in a rigid board.

Adhesive and Coverlay Layers

A multilayer flex contains adhesive layers, and an adhesive absorbs heat differently from the film. A laser that is set for polyimide can leave a reflowed adhesive edge that is wider than the film edge, and the difference produces a ledge that traps chemistry at the next step.

The parameter set is therefore developed for the stack rather than for one material. A change of adhesive supplier is a process change for the laser even when the film is unchanged.

Edge Quality and Its Measurement

The edge is judged by the width of the affected zone, by the taper through the thickness and by the absence of a recast layer. Those are measured on a section rather than judged by eye.

The measurement is worth keeping against the laser parameters, because a change in the optics or in the assist gas shows up in the zone before it shows up in the yield. Our board quality notes describe how the finished edge is judged.

Drilling Small Holes in Thin Films

A laser drills a hole in a film by removing material from one side, and the hole that results has a taper. For a microvia the taper is acceptable and for a through hole in a coverlay it is not, because the hole size on the pad side is what the solder sees.

Where the coverlay opening has to be a defined size, the parameter set is developed to place the exit side at the pad and the taper is accounted for in the artwork. That is a design decision as much as a process one.

Handling and Static

The film is thin and light, and it is held by vacuum rather than by clamps. That means the handling before and after the laser is the main risk, because a film that has been creased will not lie flat and will cut out of position.

Static is the second risk, because a charged film attracts particles and can move on the stage. Ionisation at the machine and a controlled storage between steps are the usual answers.

When Not to Use a Laser

A laser is slower per part than a die for a high volume shape, and its cost is in the machine time rather than in the tool. Where the volume justifies a hard tool and the material tolerates the mechanical stress, the die is the cheaper route.

The decision is therefore about volume and about the tolerance the edge needs. A punch produces a clean edge at a lower cost and it needs a tool that only makes sense above a certain quantity.

Process Control and Verification

The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record.

FAQ

Is a carbonised edge a defect? Where it is conductive and close to a conductor it is. The acceptance criterion is a measured zone width rather than a colour.

Can a laser cut the copper as well as the film? It can, and the parameters for copper are different, so a stack that contains copper is processed in more than one pass.

What does gopcb provide for laser processing of flex? We provide wavelength and parameter selection for the stack, zone width measured on a section, taper controlled for the feature in question, handling and ionisation control for thin films, and records that tie the parameters to the measured edge.