ESCC2006 European Supply Chain Convention

ESCC2006 European Supply Chain Convention PCB Paper 20 Laser Technology for cutting FPC s and PCB s Mark Hüske, Innovation Manager, LPKF Laser & Electronics AG, Germany

Laser Technology for cutting FPCs and PCBs Dr.-Ing. Marc Hueske, Tel.: +49 (0)5131 7095 625, m.hueske@lpkf.de, LPKF Laser & Electronics AG, Osteriede 7, D-30827 Garbsen 1 Introduction There are many issues to be considered in the manufacturing of state-of-the-art electronic products. Today's electronic devices, whether based on flexible (FPC) or rigid (PCB) printed circuit boards, require higher density and tighter tolerances due to the ever increasing demand of miniaturization and function integration. Depaneling of modern circuits requires that sensitive components are not damaged, close tolerances are maintained and contamination caused by conventional mechanical techniques is avoided. Flex and rigid-flex printed circuit boards are increasingly used offering the ability to resolve three dimensional structural issues and high density electrical interconnection. Flexible materials are extremely difficult to handle in manufacturing. Mechanical stress placed on flexible or rigid substrate materials by mechanical routing or punching equipment is disadvantageous with regard to accuracy, burr formation and reliability. LPKF Laser & Electronics AG has developed and qualified laser technology based on two different laser processes, UV and CO 2 laser cutting, meeting today's challenges in the singulation of printed circuit boards. UV laser cutting is predominantly used for flex and flexrigid singulation, CO 2 -laser cutting for depaneling rigid PCBs. Non-contact processing with a laser means no mechanical stress on the flex or rigid board or its components, no burr or debris and no extra costs for tooling. Smallest tool size of a focussed laser beam is equivalent to highest precision allowing for component placement closer to the edges of a board and increasing the net usable area on a panel. 2 Market Requirements 2.1 Flex Circuits The key of FPC manufacturing and hence success is the control of production techniques. Quite simple techniques in theory, including the separation i. e. cutting of single circuits within a panel, may prove very difficult in operating. Integrating new and innovative manufacturing techniques and automation can contribute to increase quality and yield and to keep pace with product requirements and the technological evolution. Aside from this time to market becomes one of the most important issues for FPC manufacturers. Being able to quickly react to any layout changes in prototyping and pilot production runs by applying stateof-the-art-technology can be a further key to success. Based on this laser technology nowadays is more and more used for cutting FPCs. Especially in prototyping and pre-series production expensive tooling and extensive timeconsuming manufacturing can be eliminated at the same time guaranteeing excellent quality and yield. 2.2 PCB depaneling In PCB depaneling in the first place the mechanical stress placed on the board by conventional mechanical cutting methods is the driving force behind the increasing efforts to develop and adapt laser technology. At the same time requirements on accuracy and process speed are increasing. The high positioning accuracy of a laser system allows for

component placement closer to the edge of a board. The laser beam being a small and precise tool is able to cut intricate shapes and also increases the net usable area on the panels. Further restrictions of conventional cutting methods that forward the integration of laser cutting are limitations with regard to layout freedom, an increased amount of process dusts as well as high adapter costs. 3 Laser Cutting The processing speed of laser cutting and the resulting quality depend on both, the characteristics of the material being processed and the nature of the laser emission (wavelength, fluence, peak power, pulse width, and pulse rate). It is important to know the absorption characteristic of the material to be cut. Most insulators are absorbing radiation in the UV and far IR region (compare Fig. 1). Q-Switch frequency-tripled diode -pumped Nd:YAG lasers (UV-DPSS) emitting a wavelength of 355 nm are the right choice in the UV region, producing short optical laser pulses in the range of a few ten nanosecond pulse lengths with a peak power in the range of a few kilowatts. 100 Absorption [%] 80 3xNd:YAG Nd:YAG CO 2 60 Metalls (Au, Ag, ) Insulator 40 Transition metals (Cu, ) 20 0 0.1 0.2 0.3 0.5 1 2 3 5 10 20 [µm] Figure 1: Absorption of metals and insulators Within the far IR region the laser radiation of CO 2 lasers with wavelengths of e. g. 10,6 µm is strongly absorbed. However, IR lasers remove material by intense local heating and thus CO 2 cutting is likely to leave carbonization and residue on the substrate. UV lasers on the other hand have a small average power which limits the throughput and the maximum material thickness that can be cut. In any case an improved cutting quality can be achieved by reducing the thermal influence on the material to minimize the heat affected zone and charring or carbonization respectively. Polyimide is used for flex circuits due to its high thermal and chemical stability. Polyimide does not have a melting point. The material sublimates during laser processing. Based on its relatively low evaporation temperature of 750 C a relatively low laser power is needed to achieve high cutting speeds. In case a UV laser is used the absorbed laser energy in the material is confined to a small and defined volume resulting in a high temperature gradient. This produces a defined material removal and excellent cutting quality with a minimum of carbonization. Cutting rigid PCB material is more challenging. Rigid PCB substrates consist of glass fibre bundles embedded in resin. The melting temperature of glass is extremely high, well above 1000 C. Thus the power of a laser beam needed to melt and evaporate the glass fibres needs to be high. The resin on the other hand is easily evaporated. Its melting temperature is low. The critical aspect in cutting FR4 is the different thermal characteristic of glass and resin. The existing high temperature associated with melting and evaporating the glass fibres in turn has a strong thermal influence on the resin.

4 FPC Coverlay and Body cutting Laser cutting is used in the following two process steps within the prototyping and pre-series production of FPCs: coverlay cutting for prototyping and pre-series 1 body cutting for prototyping, pre-series 2, and low volume series. Figure 2: Polyimide coverlay cut using laser technology Coverlay is one of the major differences between flexible printed circuits and rigid circuit boards. Coverlay is the mechanical protector for the fragile conductors on flex circuits and dictates solder coating areas (apertures) for component assembly 3. Usually, the same films are used as base material of coverlay and substrates. The major problem in manufacturing the coverlay films for FPCs is dimensional distortion during the lamination process, which makes automation difficult and increases costs. Apertures into the coverlay films are usually introduced prior to lamination. Coverlay films comprise polyimide with a thickness of either 12,5 µm or 25 µm coated on one side with adhesive attached to release paper. Body cutting designates the singulation of flexible circuits from their panel. Usually this comprises the cutting of arbitrary shapes into a combination of polyimide layers with adhesives (single or double side, double access, multi-layer). Around connectors copper layers and stiffeners (e. g. FR4 or Polyimide) have to be cut in addition. 5 The LPKF MicroLine Series Figure 3: MicroLine 350D (left) as well as 350 Ci (middle) and its scanner based cutting head LPKF has designed the MicroLine Series for the singulation of flex, flex-rigid and rigid printed circuits. To achieve high cutting speed, accuracy and quality the systems are based on a scanner based beam deflection in a smaller area as well as a fast and dynamic x-y-table moving the sheets and panels into the working area of the scanner. The scanner comprises two galvanometer mounted mirrors used in a vector-scanning configuration to direct the 1 until mass production tools are ready 2 until mass production tools are ready 3 Covercoats or solder masks shall not be discussed here

focused laser beam across the material surface in a cut pattern created by CAD/CAM software. As such being nearly inertia-free extremely high acceleration and high moving speeds are possible. Accurate x-y-theta alignment of the laser focal spot is achieved through a CCD-camera based vision registration systems. An automatic calibration by means of a special sensor removes the effects of thermally induced drift of the scanner and eliminates laser power variations. A honeycomb vacuum table holds material of arbitrary shape, variable thickness materials, no fixtures are needed. LPKF uses a dust and particle extraction and filtering to prevent any contamination of the workpiece or the environment. LPKF CircuitMaster software supplied with the laser systems of the LPKF MicroLine Series provides easy to understand, flexible and intuitive system control. The software controls all process parameters so new materials and processing techniques can be accommodated. The systems are also available in a sealed housing (standard for the 350 Ci) designed to be integrated into an existing production line. The LPKF MicroLine 350D is designed to cut standard format coverlay films, flex and flexrigid circuit. It is equipped with a state-of-the-art diode-pumped UV laser source. The LPKF MicroLine 350 Ci is designed for cutting rigid PCB substrates and boards. It is equipped with a high average power CO 2 laser source with very good beam characteristics to achieve a small cutting kerf. Vision registration, highest position accuracy of the x-y-table coupled with a small focal spot size allow both systems to achieve an accuracy as high as +/- 20 µm. The systems' control software supports the user by supplying pre-defined parameter sets, which are based on the substantiated process knowledge of LPKF's application engineers. 6 Cutting Results and Examples 6.1 FPC effect. Cutting Speed [mm/s] 450 400 350 300 250 200 150 100 50 0 0 50 100 150 200 Thickness [µm] Figure 4: Effect. cutting speed in Polyimide sheets as a function of material thickness To evaluate the performance of the laser system a good benchmark test is to cut straight lines into polyimide sheets of different material thickness. During testing the cutting quality was taken into account, i. e. the aim was to maximize speed and cutting quality (equivalent to minimizing carbonization) at the same time. Thus the shown figures do not represent the maximum achievable cutting speed but realistic figures with regard to what is accepted by customers in terms of cutting quality. The cutting speed in flex and flex-rigid circuits will be lower since such circuits comprise numerous polyimide sheets bonded with adhesives and even stiffeners (e. g. FR4) and copper layers have to be cut around connectors. As can be seen from Fig. 4 the maximum effective 4 cutting speed possible for high-quality cuts in 25 µm material is as high as ca. 350 mm/s. Even for thick materials the achievable cutting speed is high with 42 mm/s. Aside from optimizing parameters like pulse repetition rate, average power, pulse overlap and so on the cutting strategy applied (single-pass or multi-pass) is a vital parameter to a achieve a good quality cut. The special challenge is to find an optimum parameter set. 4 the effect. cutting speed is calculated by dividing the preset speed over the number of passes

In the following various application examples will be shown covering coverlay as well as flex and flex-rigid circuits. All results show an excellent cutting precision, no burr and no to minimal carbonization. a) Coverlay 12,5µm PI + 25µm adhesive b) Transition area between flex and rigid part Figure 5: Application examples of UV Polyimide cutting c) Connector of an FPC 6.2 PCBs eff. Cutting Speed [mm/s] 500 400 300 200 100 0 0 0,5 1 1,5 2 achieve a good quality cut. Material Thickness [mm] Figure 6: Effect. cutting speed in FR4 as a function of material thickness Again the performance of CO 2 laser depaneling has been demonstrated cutting FR4 substrates of different thickness. Quality with regard to carbonization was taken into account, i. e. the curve does not represent the maximal achievable cutting speed. As can be seen in Fig. 6 the maximum cutting speed for 1 mm thick FR4 is as high as 87 mm/s. The optimization of cutting parameters is vital to achieve a good quality and performance at the same time. Since CO 2 cutting is based on a strong local heating of the material the cutting strategy applied (single-pass or multi-pass) is even more important to a Finally extensive testing to qualify CO 2 laser cutting by means of a specific test board has been performed comprising electrical strength at high voltage, solderability, shorts, alteration of electrical characteristics of the circuit, temperature influence on components placed near the cutting kerf, ageing in damp heat, mechanical stress measurement, wetting behaviour etc. All these test were successfully passed. The following pictures show some application examples. Based on the nature of CO 2 laser cutting little carbonization is always existent, but without effects on subsequent process steps. As can be seen from Fig. 7 b) and c) the cutting edge can be placed directly adjacent to copper tracks or near SMD components. The cutting edge angle that can be achieved is smaller than 10.

a) body cut in 1 mm FR4 b) cutting edge, cut adjacent to copper track on top layer Figure 7: Application examples of CO 2 FR4 cutting c) cutting edge close to SMD component 7 Summary Laser cutting, whether CO 2 or UV laser based, offers the same advantages as any other vector-based technology. Furthermore, being a non-contact tool, the laser completely eliminates mechanical stress on the material. Burr formation or micro-cracking in solder resist are avoided. Due to the small beam diameter only a small volume of material is removed. In combination with the nature of the laser ablation process, i. e. the evaporation of the material, deposits on the circuits are significantly reduced. The ability to cut complex shapes by applying a stress-free process in combination with an extremely small tool diameter allows for more circuits on a single panel increasing the net usable area, offering an unmatched flexibility. The achievable accuracy of laser cutting rigid-flex boards is significantly better than that of any other conventional technology. At the same time laser cutting offers economic advantages. Tooling costs and associated lead times are inexistent. Laser based production can start on the same day directly based on the customer's data, no waiting for tooling (cutting die or routing adapter) is necessary. Especially the FPC manufacturer is able to instantly react to layout alterations. Since the market requirements with regard precision, burr formation and lead times in prototyping and pre-series production are getting tighter conventional technologies are no longer the optimal choice. Here laser cutting is the right answer, providing stress-free cutting, short changeover times, no shape limitations, reduction of tooling costs and higher precision. PCB depaneling strongly benefits from stress-free cutting and superior cutting performance compared to conventional cutting techniques.