F L E X F L E X. Each year worldwide flex circuit demand increases, due primarily LA S E R PRO C E S S I N G O F. Polyimide requires special methods,

Transcription

1 Polyimide requires special methods, LA S E R PRO C E S S I N G O F F L E X F L E X BY SRI VENKAT Each year worldwide flex circuit demand increases, due primarily to the advantages flex offers in space consumption, weight, and flexibility. These properties are a direct result of the materials used in flex construction. Consequently, laser machining flex circuits and drilling microvias requires special consideration. High-density flex circuits are a subset of the larger flex market and are generally defined as having less than 200-mm pitch traces or less than 250- mm diameter microvia holes or both. Applications for high-density flex abound: telecommunications, computers, ICs, and medical products. Table 1 shows the required high-density flex features of several applications. Its unique characteristics make flex a necessary replacement for rigid circuits and conventional wiring schemes in a number of applications and have facilitated the development of many new applications. The largest gro w t h a rea for flex circuits is interconnecting computer hard disk drives (HDD). As the head of the HDD scans back and forth across the rotating disk, flex circuits have replaced wires in connecting the moving head assembly to the c o n t roller board. By employing a technique called flex-on-suspension (FOS), manufacturers are able to increase yield and reduce assembly costs. In addition, wireless suspensions have better shock resistance, which impro v e s p roduct re l i a b i l i t y. Interposer flex is another high-density application in HDDs, where it is used between the suspension and the contro l l e r. 28 PC FAB

2 but must s p e e do r q u a l i t ybe compro m i s e d? Newer IC packaging technologies are a second high-growth application area for flexible circuitry. Chip-scale packaging (CSP), multichip modules (MCM), and chip-on-flex (COF) all employ flex circuits. CSP interposers represent an especially large market because they are used for semiconductors and flash memory in PCMCIA cards, disk drives, personal digital assistants, mobile phones, pagers, camcorders, and digital cameras. Liquid crystal display (LCD) monitors, membrane switches that use polyester flex, and inkjet printer cartridges are three other high-growth application areas for high-density flex. Portable devices such as cell phones hold enormous potential for flex technology. This is a natural application because these devices must be compact and lightweight in order to appeal to consumers. Other emerging applications are flat-panel displays and medical devices, for which designers are able to reduce the size and weight of products such as hearing aids and medical implants. The enormous growth in all these areas is driving an increase in flex production around the globe. For example, annual HDD shipments are forecasted to reach 345 million units by 2004, about twice as much as in 1999, and conservative estimates have annual mobile phone sales reaching 600 million units by As a result, high-density flex circuit shipments are expected to increase by approximately 35 percent per year, reaching 3.5 million sq. m by Such demand requires efficient, cost-effective technologies, such as laser processing. Lasers perform three main functions during the manufacture of flexible circuits: machining (cutting and excising), skiving, and hole drilling. As noncontact tools, lasers can deliver high-intensity (e.g., 650 mw/sq. mm) light energy to a very small (e.g., 100 to 500-µm) focal point. This energy can be used to cut, drill, mark, weld, scribe, and for a variety of other materials processing functions. The rate of material processed and the resulting quality depend on the characteristics of the material being processed and the nature of the laser emission (wavelength, fluence, peak power, pulse width, and pulse rate). Flex processing uses ultraviolet (UV) and farinfrared (FIR) lasers. The former are typically either excimer or UV diodepumped solid-state (UV-DPSS) lasers, while the latter are typically sealed CO 2 lasers. Machining. The versatility and precision of laser processing makes it ideal for machining flex. Focused laser beams from either CO 2 or DPSS lasers permit materials to be processed in a direct-write fashion. By mounting mirrors on galvanometers, the focused laser beam can be directed to any location on a work surface (Figure 1). This vector-scanning technique uses computer numeric control (CNC) of the galvanometers and CAD/CAM software to create the cut pattern. This soft-tooling approach permits instantaneous and economical control of design changes. Another significant advantage of lasers is their ability to precisely register the pattern by adjusting for shrinkage and expansion and tooling inconsistencies. Vector scanning can be used to cut through flex substrates such as polyimide film, to excise entire circuits, or to remove specific features such as slots and squares. During the machining process, the laser beam remains on as the mirrors scan over the work surface to be machined. This is in contrast to the drilling process, in which the beam is only turned on when the mirrors are stationary at each drilling location. Skiving. Skiving, in industry jargon, refers to the machining process in which the laser removes one layer of material from another layer beneath

3 TABLE 1. High-Density Flex Circuit Features by Application A p p l i c a t i o n Minimum Lines/Spaces M i c rovia Diameter Minimum Inner Lead Pitch Polyimide Thickness Copper Thickness Hard disk drive 50/ , 18 w i re less sus p e ns i o n LCD driver 25/ , 75 12, 18 Mobile phones 75/ Hearing aids 50/ (+NiAu) Chip-scale packages 25/ All dimensions in mm. Source: TechSearch International Inc., Aug FIGURE 1. Vector scanning uses computer control of galvanometer-mounted mirrors and CAD/CAM software to create a cut pattern or drill pattern. A telecentric lens system is used to ensure that features are drilled perpendicular to the work surface. it. This is an operation ideally suited for lasers; the same vector scanning technique is used to remove dielectric material and expose buried conducting pads. Again, the precision of laser processing is a great advantage. Because FIR laser emissions are reflected by copper, CO 2 lasers are predominantly used in this case. Drilling. Although methods such as mechanical drilling, punching, and plasma etching are still used, laser drilling is the most widely used microvia formation technique for flex. The primary reasons for the widespread use of laser drilling are productivity, flexibility, and maximum uptime. Mechanical drilling and punching machines use precision drill bits and dies to generate holes to approximately 250 mm in diameter in conventional flex circuits. These precision tools are expensive and have relatively short lifetimes. Because high-density flex requires via diameters less than 250 µm, mechanical drilling is not considered a viable option. Plasma etching can produce microvias with diameters smaller than 100 mm on 50-mm thick polyimide substrates, but the capital equipment costs associated with both processes are extremely high. Plasma etch is an expensive process to maintain, too, especially in regards to chemical waste treatment and the cost of consumables. Furthermore, it takes a significant amount of time to develop new processes that consistently yield reliable microvias. The advantages of plasma technology are higher reliability and reported yields of 98 percent in microvia formations. This finds niche applications in medical and avionics. In contrast, laser drilling microvia holes is a straightforward and comparatively inexpensive process. The capital equipment costs are far lower, and because a laser is a noncontact tool, there is no expensive hard tooling to replace, as in mechanical drilling. Furthermore, modern sealed CO 2 and UV-DPSS lasers are maintenance-free, which minimizes downtime and maximizes productivity. The methods used to drill microvia holes in flex are the same as those used for drilling rigid PCBs, but because of diff e rences in substrate materials and thickness, critical laser parameters must be modified. Both sealed CO 2 and UV-DPSS lasers can directly drill flex using the same vector scanning technique used to machine parts mechanically. The only diff e rence is that in drilling applications software turns off the laser beam as the scanning mirrors scan from one microvia location to another; the laser is turned on only at the next drilling location. In order to drill holes that are perpendicular to the surface of the flex substrate, the laser beam must also strike the substrate 30 PC FAB

4 p e r p e n d i c u l a r l y. This is accomplished by placing a telecentric lens system between the scanning mirrors and the work surface (Figure 1). Conformal mask techniques can also be used to drill microvias with a CO 2 laser. With this technique, the copper surface is used as a mask in which holes are first etched using standard printing and etching methods. A CO 2 laser beam is then used to illuminate the holes and remove the exposed dielectric material. Excimer lasers also use projection masks to cre a t e m i c rovias. This technique re q u i res that the image of either a single microvia or an entire array of microvias be pro j e c t e d onto the substrate. Once the excimer beam illuminates the mask its image is projected onto the surface and the hole is drilled. Excimer lasers offer quality advantages, however, lower speed and higher costs of operation limit this technology. FIGURE 2. UV laser-drilled holes in Kapton. SELECTING THE LASER Although the types of lasers used to process flex are the same as those used to process rigid PCBs, differences in substrate materials and thickness can greatly affect processing parameters and speeds. Excimer and transverse excited atmospheric (TEA) CO 2 lasers are used in some cases, but their low processing rates and high maintenance costs limit productivity. In contrast, because of their versatility, higher speed, and lower cost, sealed CO 2 and UV-DPSS lasers are the two primary types of lasers used to drill microvia holes and machine flex circuits. Sealed CO 2 lasers feature slab-discharge technology, which in contrast to flowing-gas CO 2 lasers, confines the laser gas mixture within a laser cavity defined by two rectangular plate electrodes. The laser cavity is perm a n e n t l y sealed for the duration of its lifetime, usually two to three years. Sealed lasers are compact, re q u i re no replacement gases, and the head re q u i res no maintenance for up to 25,000 hours of continuous operation. A significant advantage of the sealed design is the fast pulsing capability. Slab-discharg e lasers emit high-frequency pulses (100 khz) with up to 1.5 kw of peak p o w e r. The combination of high frequency and high peak power results in faster material processing with minimal thermal degradation. UV-DPSS lasers are solid-state devices that use diode laser arrays to continuously pump a crystal rod of neodymium vanadate (Nd:YVO 4 ) and an acousto-optic Q-switch to generate a pulsed output. Third-harmonic generation crystals are used to shift the output of Nd:YVO 4 lasers from the IR fundamental wavelength of 1,064 nm down to the UV wavelength of 355 nm. The average output power of a typical 355 nm UV-DPSS laser is above 3W at a nominal pulse repetition rate of 20 khz. CO 2 PROCESSING Sealed CO 2 lasers are available with FIR wavelength emissions at either 10.6 µm or 9.4 µm. Even though both wavelengths are readily absorbed by dielectrics such as polyimide, studies show that the 9.4-µm wavelength is clearly superior for processing these types of materials. Because the absorption coefficient of dielectrics is higher at 9.4 µm, they can be cut or drilled far more quickly than at 10.6 µm. A 9.4-µm wavelength is superior during the drilling and cutting processes as well, but its advantages are especially obvious during skiving. Consequently, using the lower wavelength output 32 PC FAB

5 TABLE 2. Polyimide Processing Speeds for a Typical 9.4-µm Wavelength CO 2 Laser Process Polyimide Thickness Nominal Processing Speed Machining/cutting 50 µm 500 mm/sec. Microvia drilling 100 µm 100-µm diameter via, 450 holes/sec. Microvia drilling 50 µm 25 to 50-µm diameter via, 400 holes/sec. TABLE 3. Processing Speeds for a 3-watt, 355-nm Wavelength UV-DPSS Laser Process Material Thickness Processing Speed Machining/cutting polyimide 50 µm 50 mm/sec. Microvia drilling 50 µm 30-µm diameter via, 100 holes/sec. (copper + dielectric) Direct copper drilling 50 µm 30-µm diameter via, 100 holes/sec. increases both productivity and quality. In general, FIR wavelengths are absorbed by dielectrics but reflected by copper. Consequently, the vast majority of flex processing applications for CO 2 lasers involve machining, skiving, and drilling dielectric substrates and laminates. Because CO 2 lasers have higher output powers than DPSS lasers, most dielectric processing is carried out using CO 2 lasers. As a result, CO 2 and UV-DPSS lasers are frequently used in combination, such as in drilling microvias, where a DPSS laser removes the copper surface and a CO 2 laser quickly drills the dielectric down to the next copper level. With their inherent short wavelength, UV lasers produce a finer spot size than CO 2 l a s e r s. In certain applications, however, the larger focal spot diameter of CO 2 lasers is an advantage over U V-DPSS lasers. For example, when tre p a n n i n g to remove larger features such as slots and s q u a res or when drilling larger diameter vias (>50 µm), overall processing time is faster with CO 2 lasers. Generally, CO 2 lasers are better suited for processing features in excess of 50-µm diameter, while UV-DPSS lasers are better suited for features less than 50-µm in diamet e r. Table 2 shows typical polyimide processing speeds for a CO 2 l a s e r. The faster processing speed shown for drilling microvias in polyimide of 100- µm-thick compared with that of 50-µm-thick is due to the slow, high-pre c i- sion positioning accuracy re q u i red to drill 25 to 50-µm diameter micro v i a s. REFERENCES E. Jan Vardaman, Flexible Circuits for High Density Applications, TechSearch International Inc., Aug SRI VENKAT is business development manager for the Advanced Packaging and Interconnects business unit of Coherent Photonics Group Laser Division (Santa Clara, CA). He can be reached at 408/ ; sri_venkat@cohr.com. UV-DPSS PROCESSING Both dielectrics and copper easily absorb the 355-nm output wavelength of UV-DPSS lasers. In dielectric processing applications, because of an inherently small focal spot size and lower output power compared with CO 2 lasers, UV-DPSS lasers are usually used to process features with small (<50 µm) dimensions. As a result, UV light is ideally suited for drilling smaller than 50-µm diameter microvias in high-density flex substrates. As new, more powerful UV-DPSS lasers become available, there will be a potential increase in UV dielectric machining and drilling speeds. An advantage of UV-DPSS lasers is that their high-energy UV photons directly break molecular bonds at the surface layer of many nonmetals in a cold photo-ablation process that produces features with smooth edges and minimal thermal damage or charring. Thus, UV micro-machining is preferable in more demanding applications where post-processing is either impossible or undesirable. Materials with high UV ablation thresholds such as copper are processed at high energy with low repetition rates. Low-threshold dielectric materials like polyimide dielectrics are processed at lower energy and higher repetition rates, both to avoid damage to the copper pad and to maximize throughput. To increase throughput, most large-diameter microvias are drilled in a two-step process, with UV-DPSS lasers direct-drilling the copper surface and CO 2 lasers drilling the exposed dielectric material. Table 3 shows typical processing speeds for a UV-DPSS laser. Comparing Table 2 and Table 3 clearly shows the advantages of each laser. PC FA B 34 PC FAB