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Laser Machining of Flexible Printed Circuits

Lasers have been used to process flex circuits for over 15 years.  The current requirements for precision processing of parts for the hard disk drive industry (suspension flex) and the electronics packaging industry (chip on flex, multi-chip modules, PCMCIA cards) are typical of the applications where lasers provide manufacturers with significant advantages. 

The choice of a laser and beam delivery system for any of these processes is strongly linked to the type of features, size of features, the quality of the process (e.g. lack of process debris, edge quality), the patterns that are required on the part, and the number of parts that will be required, among many other factors.

Laser Sources

It was established in the early 1980’s that the ultraviolet output from excimer lasers was an excellent source to machine polymer films.  Ablative photodecomposition, or ablation for short, was characterized by an energy density threshold on the material, beyond which the polymer chain would be fragmented.  These fragments would explosively leave the interaction region, and the hot products would combust in air, generating the well-known ablation plume.  Depending on the energy density of the beam on the part, a given depth of material (typically tenths of a micrometer) would be removed with each laser pulse.  Repeated pulses would then etch through the material.  The UV ablation provided very clean side walls on the polymer, with little or no thermal damage, and the process would self-terminate on a metal surface with little residue, quite often solderable ‘as is’.

The beam from an excimer laser is typically rectangular in shape, with a ‘top hat’ intensity profile.  Such beams are best utilized in parallel processing of many features simultaneously through the technique of mask imaging.  The features to be produced are first generated in a metal mask, which is then placed in the beam.  A lens is used to produce a demagnified image of the mask pattern, allowing very small features to be generated in the target material.  Since the image is demagnified, the light intensity is increased to values well above the ablation threshold, providing very clean sidewalls.  Arrays of holes can be generated by stepping and repeating this pattern.  Slots can be generated by focusing the beam to a line, and translating the part underneath the beam at an appropriate speed.  Lumonics produced a very large step-and-repeat system for a customer using a sophisticated beam delivery employing diffractive optical components.

In the 1990’s, Lumonics introduced the IMPACT™ line of TEA-CO₂ lasers.  These lasers operated in the 9μm band, which had been found to provide effective ablation of many key polymers.  The process also self-terminates at a copper base layer, since copper is highly reflective in the mid-IR.  While the process quality is not as clean as the excimer laser, it is acceptable to many applications, and the TEA-CO₂ laser has higher ablation rates (greater depth can be removed per pulse) and significantly lower operating costs than a comparable excimer laser.

The beam from the TEA-CO₂ laser is also a top hat profile, and so the beam can be used in a similar mask imaging fashion to the excimer laser.  This is most appropriate for higher energy beams, while lower energy, higher repetition rate lasers can also be used in a focused spot mode. 

Since both the excimer and TEA-CO₂ lasers are most efficiently used in a mask imaging mode, the material is moved underneath a fixed beam location with XY motion stages.  However, in this situation, processing of random patterns of holes or slots is typically inefficient with regard to both beam utilization and time to move from feature to feature.  Another limiting factor is the repetition rate of these lasers, in the range of 100-500 Hz.

Fortunately, alternative, complementary laser sources have become available.  RF-excited CO₂ lasers produce low energy, high repetition rate pulses in a Gaussian profile beam.  Such devices have also been made to operate in the 9μm band, providing good absorption in key material such as polyimide.  Rather than area processing, the beam is used in focused spot mode, so that the limited pulse energy can exceed the energy density of the ablation threshold.  The high repetition rate then allows high drilling or cutting rates.

Developments in diode-pumped solid state lasers have generated similar low energy, high repetition rate sources in the UV region.  While the average powers are significantly lower than excimer lasers, the excellent Gaussian beam quality (allowing very small focal spot sizes) and high repetition rates enables very efficient processing of very fine circuit features in flex panels.  The shorter wavelengths available from the UV lasers (excimer and DPSS) allow for smaller features than can be generated by the longer wavelength CO₂ lasers.

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Beam Deliveries

As mentioned above, large top-hat beams are most efficiently used to process large areas of material on a pulse-by-pulse basis.  The more holes that can be imaged onto the area defined by the optimum energy density, the higher the effective drilling rate.  An XY table set is used for the step-and-repeat positioning of the material under the imaged beam location.

Applications

Drilling

Both through and blind holes are possible, since at the process conditions to ablate polyimide, both the UV and CO₂ beams will be reflected from underlying copper.  As mentioned above, fixed arrays of holes can be processed in parallel using an excimer or TEA-CO₂ laser source and mask imaging beam delivery.  Processing parameters for ablating polyimide are very well understood.  The nature of the light-matter interaction will produce a slight taper to the hole, such that the exit hole is smaller in diameter than the entrance hole.  The taper can be reduced through the use of higher energy densities, but this restricts the area that can be processed with each pulse, and in the extreme will generate the risk of damage to underlying copper if drilling blind holes.

Only a few applications have fixed, small patterns that are suited to laser etching with a large beam.  Most hole patterns are dispersed across the sheet or web, and so drilling must occur on a hole-by-hole basis.  While the excimer and TEA-CO₂ lasers can be used in this way, the bulk of their outputs will be wasted at the mask.  Both the RF-CO₂ and UV DPSS lasers are suitable laser types to consider for drilling one hole at a time.  If the hole diameter is larger than can be covered by the available pulse energy, then the hole must be ‘drilled’ by a spiral or trepanning motion of the small beam over the larger area, slowing the drilling rate.  This can be accomplished by moving the workpiece with a sub-routine for the XY tables if necessary, but is much more efficiently accomplished by moving the beam with galvanometers.

While the UV lasers provide cleaner hole quality with better edge definition, customers have proven time and again that the CO₂ lasers offer adequate hole quality.  Typical post-processing cleanup consists of either a chemical desmear or a quick plasma etch, and is standard in most flex shops.

Drilling can also be accomplished through the use of a contact or conformal mask which has been pre-etched with the desired pattern.  The laser then floods the opening in the mask, ablating any exposed polyimide.  This is commonly used in the rigid board industry for microvia holes.  In this case the UV DPSS laser can be used to create the openings in a conformal copper layer (at a much higher energy density than would be used to process polyimide), and the CO₂ laser used to most effectively remove the polyimide.

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Cutting

Several applications are covered under this category.  The laser beam can be used to cut a slot through the material, or a blind trench down to a copper stop layer.  The extreme case of cutting a through slot is the excising of a part from the panel.  The optimum laser  for the application depends on the complexity of the shape required, and again the desired edge quality.  The smaller the parts being excised, the greater attention is typically paid to the heat affected zone at the edge of the part.  If several parts can be fit under the field of a galvo lens (e.g. small suspension flex from a large panel), then the galvo beam delivery will likely lead to higher throughput.  The RF-CO₂ laser has proven to be the laser of choice for the majority of cutting applications involving just polyimide and adhesive layers (i.e. no metal).  UV lasers are chosen when there is a requirement to cut through copper as well, as it will easily cut the dielectrics at the process conditions to cut the copper.

Skiving

The process of removal of large areas of material is referred to as skiving.  The use of the word ‘large’ is, of course, relative.  If the areas are larger than hole diameters, then the small Gaussian beams must be directed in a raster pattern across the material.  Small to medium size areas can be covered in this way reasonably efficiently, especially if galvo-based beam deliveries can be employed.  This is especially true if the area to be skived is irregular in shape.

A typical PPI system

In order to process a panel of flex circuits, several building blocks need to be successfully integrated in order to provide the capability for performing all three functions (cutting, drilling and skiving).  The system integrator needs to select the correct laser source, the beam delivery to deliver the correct process conditions, and part hold-down and motion. 

The laser source for a typical system for flex machining would comprise an RF-CO₂ laser with acousto-optic modulator (AOM).  This source will provide high peak powers for clean ablation with small heat affected zone, high repetition rate to achieve high throughput, and controlled pulse energy for automated processing of a panel with different areas having different process requirements.  This laser will provide a fast clean process for cutting and skiving of cover films and polyimide in general, whether for through or blind features.  By contrast, a UV DPSS laser is a better choice to expose delicate copper fingers, and for multilayer copper/polyimide cutting.

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The sophisticated motion controller is interfaced to the operator through a PC running a Windows® environment.  Operator access is limited to the basic commands required to load and run a pre-set job.  Administrator access is password protected and allows creation of job files as well as service and calibration settings.  Alignment to fiducial marks on the panel is achieved through an integrated vision system.  Panels can be loaded manually, or an optional autoloader can be provided.

It should be noted that these laser sources can, under appropriate conditions, also remove thin metal films from polymer substrates, quite the opposite of their major application.

Clearly there are many factors contributing to a successful, efficient laser process.  Consult the team at Process Photonics to have their years of experience help you with your process requirements.

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