6 Things You Need to Know About Laser Wire Stripping

Medical device wires are getting increasingly tinier, posing new challenges when it comes to using lasers to strip. Here are the basics to get you started in the world of laser wire stripping for medtech.

Glenn Ogura and Sergey Broude, Resonetics

It was only as recent as 20 years ago that electronic component makers turned to lasers--an elegant non-contact solution--to strip the majority of tiny wires that connected the disk drive head to the drive electronic. These days, the emergence of medical device applications in electrophysiology, neurovascular, cardiac rhythm management, and more are pushing the boundaries of conventional laser stripping technology. New technical advancements include reel-to-reel part handling with 100% real-time inspection, and closed loop end point detection to address stripping ultra-thin wires, smaller strip regions, multi-wire ribbons, braid catheters, and catheters.

This world is more sophisticated than ever. Here are six things to help get you bearings:

1. Why It Is Needed

The insulation is removed from electrical wires because every wire used for either power or signal transmission must be eventually terminated with good ohmic contact to the next circuit element such as a connector or contact pad. The methods of joining include welding, soldering or crimping but all of them require that the exposed section of the wire is void of the insulation. The degree of cleanliness and quality of the exposed metal wire depends upon the specific application. Some applications have more stringent requirements than others. Ultimately the process and system design are influenced by finding the optimal balance between the stripping throughput (generally related to the aggressiveness of the process (e.g. intensity of laser irradiation)) and the preservation of the wire characteristics (surface roughness, strength, conductivity, appearance, etc.)

For biosensor applications, polymer coatings are stripped from metal electrodes to create a clean surface where an electro-chemical reaction can be used for sensing the (bio)-chemical environment (e.g. for in-vitro diagnostics) such as detection of glucose, DNA and antibodies.

Many methods of wire stripping are widely employed in industry: abrasive, mechanical, thermal or chemical. However the life science industry presents some stringent requirements. There is a need in the medical device industry for very clean insulation removal, very gentle impact on  thin and flimsy wires down to diameters of 10 to 25 microns, and environmentally clean and safe processing.

Wires, ribbon cables, twisted wire, and single/multi-lumen catheters and braided catheters require laser stripping, either end-stripped or mid-span. In some cases, the outer layer(s) of a catheter are removed to decrease the outside diameter so that there is access to the inner lumens or to reduce the catheter stiffness. Similarly, removal of outer coils in a multi-layer shaft also can be done with lasers.

Resonetics Metal-Braided Catheter

Removal of outer polymer layer off a metal-braided catheter (Image courtesy of Resonetics)

2. Major Factors Behind Laser Stripping

Medical device makers need to consider the material type of the conductors and insulators, the
geometry of desired stripped sections, the configuration of the process part handling (reel-to-reel, reel-to-cut, individually singulated part), and the required process throughput (and cost).

Fundamentally, laser stripping can be organized so that most of laser energy is absorbed by the insulating layer, or most of the laser energy is absorbed by the conductor, or a combination of both.

Absorption of the insulating layer implies that the laser delivers enough energy to effectively remove the insulating material by either photo-thermal or photo-chemical ablation process. Photo-thermal is essentially the heating and resulting evaporation of the insulator while photo-chemical ablation describes the breaking of chemical bonds in the material. Qualitatively, the two mechanisms produce very different results.

Absorption of the conductor (wire) implies the delivered laser energy to the wire results in local heating that leads to evaporation of the surrounding insulating layer. An alternate mechanism is recoil stripping, where the insulating layer is removed by the shock wave created at the wire surface by the laser pulse.

A hybrid mechanism involves the partial absorption by both the insulating layer and conductor (wire), developed to balance the pursuit of the best quality of the stripped region versus the economics of the process in terms of throughput and ultimately cost.

For the stripping of wires with thicker insulation, a simple solution involves laser scribing around the wire in the longitudinal or radial direction and then sliding the intact insulation off the wire (in case of stripping the end of the wire) or un-wrapping it (in case of stripping in the middle of the wire). For the stripping of wires with thin insulation, this technique won't work because the insulation will not remain intact so instead, the entire material is ablated. While the ablation of all the insulating material may seem less efficient than ablating only a portion - more cubic micrometers of the material needs to be removed - full material ablation  is more amenable to automation as there is no post laser processing or  handling required.

Check out Resonetics and many other cutting-edge medical device suppliers on the show floor of MD&M Minneapolis, November 4-5, 2015.

3.  Selecting the Right Laser

Depending on the material type and application, two basic laser approaches are used:

  • You can employ a large multi-mode laser beam. A large laser beam exposes a substantial area of the wire surface. If stripping of areas larger than the available beam size is needed, a step-and-repeat or scanning is performed (usually along the wire). This method is used with a variety of laser such as TEA-CO2 lasers at wavelengths in the 9.6 to 10.6 micron range and excimer lasers operating at wavelengths of 351nm, 308nm, 248nm and 193 nm.
  • You can use a tightly-focused single-mode laser beam. A small laser beam exposes a small area of the wire surface and is scanning sequentially across entire area of the strip zone. Multiple direct write laser sources are used including RF-excited CO2  in the 9.6 micron to 10.6 micron range,  diode-pumped solid-state lasers operating at the fundamental wavelength around 1 um and the corresponding  harmonics in the green (532nm) and the ultra-violet at 355nm and 266 nm as well as ultra-fast laser operating in the green and ultra-violet wavelengths.

The most common multi-mode laser sources to strip wires are excimer lasers and TEA-CO2 lasers. The selection of the laser depends upon criteria including minimum strip length, sharpness of the transition zone, cleanliness of the wire surface, and cost.

Excimer lasers operate in the ultra-violet range (193nm to 351nm) while TEA- CO2 lasers operate at either 9.6 microns or 10.6 micron wavelengths. As a coarse rule of thumb, the minimum feature size is proportional to twice the wavelength. Taking into account the material interaction, the excimer laser can produce a strip length as small as 1 to 2 microns, measured at the wire surface. On the other hand, the TEA-CO2's minimum strip length is approximately 50 microns (0.002 in.) due to the much longer wavelength.

Due to the relatively strong material absorption of most polymers in the ultra-violet wavelength, the transition from unstripped to stripped region produced by an excimer laser is quite well-defined and sharp. Although TEA-CO2 lasers are tuned to the 9 micron wavelength to take advantage of a modest absorption peak, the transition zone is less delineated due to the photo-thermal interaction.

Resonetics Excimer-Laser-Stripped Wire
Excimer laser stripped wire (Image courtesy of Resonetics)
Resonetics TEA-CO2 stripped wire
TEA-CO2 stripped wire (Image courtesy of Resonetics)

A byproduct of the photo-chemical mechanism is that the excimer laser "vaporizes" the coating cleanly from the wire surface, leaving no residue, while the TEA-CO2 laser, in some cases, may leave an ultra-thin residue. When soldering to the stripped metal surface, the presence of any organic residue affects the wettability on the wire surface and ultimately the integrity of the joint; hence the excimer laser is the preferred choice if 100% of the coating must be removed. On the other hand, when welding or crimping to the exposed metal surface, then the additional heat or pressure is usually sufficient to produce a consistent, reliable joint. Over-pulsing of the TEA-CO2 laser does not remove the ultra-thin residue because the laser's wavelength is longer than the thickness of the coating, making the coating transparent to the incident laser beam.

TEA-CO2 laser can etch polymer coating an order of magnitude faster than an excimer laser and it also has a lower operating cost than an excimer laser. The TEA-CO2 laser has a large beam, similar to an excimer laser, and uses the mask projection scheme to illuminate the wire. Therefore the TEA-CO2 laser is a more cost-effective laser to strip wires, but the lower cost must be weighed against the quality, strip tolerances and the subsequent joining operation.

4. Choosing the Right Stripping Technique

There are two techniques to laser strip wire. You can  rotate a wire around its axis to expose all surface to one laser beam. Or you can direct multiple laser beams around the wire from several directions simultaneously. 

For applications where only a "window" of insulation needs to be removed, then a single beam or no rotation is sufficient. However automated machine vision with accompanying illumination techniques may be needed to selectively strip certain sections on a ribbon cable or twisted wire.

For life science devices needing a short laser-stripped wire such as 25mm in length, rotating the wire is easy to implement. For reel-to-reel or reel-to-cut applications involving handling a spool of wire of thousands of feet or meters, laser stripping is more challenging but met using novel part handling approaches where the laser pulses are synchronized with the fed wire directed by sophisticated part handling systems using precise motion controllers and software.

Multiple beams (2, 3 or 4 beams are common) can be generated by beam-splitting, beam-slicing and retro-reflecting techniques, taking into account the optimal utilization of available laser energy for efficient material removal in specific product configurations. Generally speaking, the more beams, the greater the overlap and more uniform circumferential coverage of the insulation to be laser-stripped.

Often there is a need to have stripped regions of vastly different lengths on the same wire or catheter, ranging from narrow strips of 10 to 25 microns wide to ones longer than 100 mm. The number of stripped regions can vary from a couple to a dozen per device. Narrow stripped regions necessitate the use of high resolution optics and selecting a laser producing minimal heat-affected zone (HAZ).  These narrow stripped regions can be created with tolerances on the length on the order of a few micrometers. Strips longer than the available beam width require scanning the beam along the length of the wire.

Resonetics Narrow 25 micron wide strip length
Narrow 25 micron wide strip length (Image courtesy of Resonetics)

In some cases, the design of a life science device or consumable requires a sharp transition between the stripped and non-stripped regions while for other designs, a gradual transition is required. Distribution of deposited laser energy can be controlled to extend the transition profile.

Resonetics Extended transition zone stripped by excimer laser
Extended transition zone stripped by excimer laser (Image courtesy of Resonetics)

In some cases the laser system can discriminate between multiple coatings on a wire. Depending on the material types, the laser can strip the outer coating without damaging the underlying layer or continue to etch to the bare wire.

5. Proper Wire Material Handling

With respect to the wire material handling, thin wires can be laser stripped reliably only when they are held straight by applying the appropriate tension. For wires and coating materials requiring gentle handling (e.g. maximum allowable tension forces on the order of 5 to 50 grams), reaching  high acceleration and velocity of wire transport is a  challenge but can be attained by advanced tooling design and sophisticated motion and tension controls.

Wires can be laser stripped in three product configurations:

  • Individually singulated: An individual wire--which has already been cut to the correct length--is laser-stripped
  • Reel-to-cut:  A spool of wire is advanced through the system, stripped at the required locations and then singulated into individual parts. It is critical that the singulation process does not create unacceptable irregularities at the ends of the part. Special mechanical or laser cutters are used for this. More often, a different laser is used for cutting than for stripping.
  • Reel-to-reel: A spool of wire is advanced through the system, stripped at the required locations and then re-wound onto a new spool.

To strip short wires, we use longer wires so that they can be tensioned, laser-stripped and then cut to the final length.  For volume applications, a reel-to-reel stripping machine can be integrated with a reel-to-cut system.

6. Employing an Intelligent Control System

The energy density to ablate a polymer with nano-second lasers is typically an order of magnitude lower than the threshold to ablate metal. Once the laser has removed the polymer insulation, additional laser pulses can be applied to the metal surface to ensure cleanliness without concern of damage. However operating the laser in an open loop fashion is not always desirable.  For example, there are multi-layer coatings where each coating is a different material type or the wire is so thin or fragile that additional laser pulses must be kept to a minimum. In the real world, the thickness of coatings around the wire is non-concentric. By implementing an intelligent closed loop control system such as Resonetics Assure End Point Detection which monitors the ablation plasma plume on each pulse, the laser is turned on and off to avoid going too deep in the thinner sections of the wire or etching too shallow in the thicker sections of the wire. This in-situ end point detection system allows the wires to be stripped uniformly and consistently, independent of the inevitable variation of the wire coating from lot-to-lot. 

One of the challenges of a reel-to-reel process is that once the laser-stripped wire has been re-spooled on the take up reel, the wire can no longer be easily inspected for the cleanliness of the stripped zone, the strip length, the transition zone between the unstripped and stripped section or the distance between strip zones. To solve this dilemma, 100% of the wire is inspected on-the-fly using high-speed, high-resolution optical imaging systems, optical rulers or fluorescence detection schemes operating under the appropriate excitation. As strip length get smaller, this in-situ inspection may be the only practical means of inspecting the wire to avoid over-handling and possibly damaging the wire.

Check out Resonetics and many other cutting-edge medical device suppliers on the show floor of MD&M Minneapolis, November 4-5, 2015.

Glenn Ogura is senior vice president of market development of Resonetics (Nashua, NH). Sergey Broude is vice president of advanced technology at Resonetics.

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