PACKAGE SINGULATION: Options in Laser Singulation
03/01/2008
BY ANDREW CHANG AND TIMOTHY EDWARDS,
The advent of pulsed lasers with short wavelength, focusable output, and high power levels presents opportunities for tool builders and end-users in package singulation. For curved profiles such as those on microSD cards, the laser has the ability for cutting these curves. For straight line cutting, the laser is becoming competitive with conventional saws. As a result, laser singulation has reached a critical point in development and is poised for rapid growth.
Until recently, package singulation needs were met with diamond saws. This method can singulate at many thousand UPH (units/hour) and handle both thin (<0.5 mm) and thicker (over 1 mm) packages, with acceptable consumable costs and uptime/downtime characteristics. Diamond sawing delivers acceptable results in terms of edge quality (surface roughness). Unfortunately, sawing is only suited to straight line cuts and is thus limited to rectangular and square packages. The advent of shaped packages has spurred the industry to find alternatives.
At first, this need was partially met with abrasive water jet (AWJ) technology, which relies on powerful pumps to pressurize water up to 60,000 psi. The water contains a suspension of garnet grit. Forcing the mixture through a small (typically 5-13 mil) sapphire orifice creates a tool capable of cutting materials as diverse as plastics, metal, glass, and semiconductors. Water jet technology delivers a smooth cut with among the lowest surface roughness of any technology. However, AWJ is not a panacea to this problem. For instance, it is limited to less than 1000 UPH, even when used only for curved cuts. It also requires post-processing in the form of washing and drying. And while the water can be filtered and re-cycled, the vigorous AWJ action reduces the garnet grit size, so this cannot be re-cycled. Other consumable costs include the sapphire orifices which enlarge with use. Additionally, the AWJ process creates a high noise level, thus requiring acoustic protection/isolation for operators. Finally, the typical system up-time for water jet technology is lower than both diamond saws and lasers.
Because of the relatively slow speed and high cost of operation of the AWJ, many manufacturers have switched to grinding. Here the package is singulated with four straight cuts and then curves are created as indents with an automated grinding tool. In the case of microSD, this hybrid method can deliver singulation rates up to 5000 UPH and excellent edge quality. But the grinding produces debris and thus requires the complexity and cost of post-process cleaning. Additionally, the grinding head deteriorates with use, incurring replacement and downtime costs.
In response to the limitations of these mechanical methods, tool builders and manufacturers of packaged products began to explore laser singulation. In turn, laser manufacturers have responded with products specifically optimized to meet the unique needs of singulation.
Photothermal vs. Photoablation
Lasers are already used for die singulation, as well as for marking wafers and packages. Now the laser has emerged as an attractive alternative to mechanical methods for singulating IC packages such as QFN, FBGA, and MicroSD (MMC) as well as direct chip attach (DCA) type packages. Often called laser sawing, this technique is particularly useful for contoured packages as well as for surface mount products. In the latter case, the laser’s ability to make narrow cuts allows electronics to be placed very close to the edge for maximum yield and minimum footprint. Moreover, laser cutting produces consistent results; it is a non-contact process and is not subject to the effects of tool wear, unlike mechanical methods.
Lasers can remove material by two different mechanisms. An infrared laser acts as an intense localized heat source. Material is heated up by excitation of molecular and lattice vibrations. When the intensity exceeds a certain threshold, the material is boiled off in a strictly photo-thermal mechanism. Not surprisingly, this type of material processing produces a lot of peripheral heating as well as loose debris, recast material, and relatively rough edges.
At the other extreme, a deep ultraviolet laser can cut organic materials (e.g. plastic, epoxy, paper) by a process called ablation. Here absorption of these higher energy laser photons excites electrons in the target material, directly breaking inter-atomic bonds. Compared to photo-thermal processing, this is a relatively cold process since there is much less vibrational excitation. Because the material is atomized, ablation is also characterized by clean, smooth edges, and the creation of little debris. However, for a given laser power level, photo-ablation is usually slower than photo-thermal processing. Additionally, for commercial lasers, the cost per watt is higher for UV lasers than visible lasers, which in turn is higher than for infrared lasers.
What does this mean for microSD singulation? As a general rule, shorter wavelength translates into cleaner, i.e. smoother, edges, but shorter wavelength means higher costs and slower speeds. Fortunately, the organic materials (polyethylene, epoxy, molding compound, etc.) in most packages begin to ablate at visible wavelengths (Figure 1). A visible laser will remove material by a combination of both photo-thermal and photo-ablative processing. With the latest commercial laser technology, a visible (green 532 nm wavelength) single-mode laser can deliver the requisite combination of high edge quality and fast throughput. In contrast, infrared lasers cannot deliver the edge quality, and ultraviolet lasers are not yet cost-competitive for this particular application.
 Figure 1. At longer wavelengths, lasers cut primarily with a photo-thermal process, whereas at shorter wavelengths, the colder photo-ablation process becomes dominant. |
Optimizing Edge Quality for MicroSD
Achieving high edge quality requires a number of laser parameters to be optimized for the exact needs of singulation. These include laser pulse duration, pulsing rate, dwell time, laser power, and mode quality.
Optimizing edge quality requires minimizing peripheral heating — the heat-affected zone (HAZ). This means maximizing the proportion of the total laser power that is delivered above the ablation threshold for the material(s). One way to do this is to use a laser with short (nanoseconds) pulse duration and pulses with fast rise time (square wave pulses) (Figure 2). The heat created by each pulse is small and has time to dissipate before the next pulse arrives. Fortunately, nanosecond pulse duration is readily achieved with frequency-doubled diode-pumped solid state (DPSS) lasers.
 Figure 2. Short (nanosecond) pulses with a fast rise time limit thermal damage. |
When it comes to limiting peripheral thermal effects, spatial confinement of the laser pulse is just as important as temporal confinement. It’s crucial to use a laser beam that can be focused to a small intense spot rather than a diffuse area. This type of spot is obtained by designing the laser to produce TEM00 single-mode output where the beam has a round cross section whose power peaks in the center. As a general rule with lasers, it becomes increasingly difficult to maintain high mode quality as power is increased. But singulation needs power levels of at least 30 watts to deliver the speed and cost-effectiveness to compete with AWJ and grinding. So the laser has to be carefully designed to deliver this high power level together with TEM00 output.
A high-pulse repetition rate is also desirable. This enables fast sweeping of the laser beam across the material without producing a dotted line cut. For package singulation, a repetition rate below 100 kHz would limit processing speed. For this reason, the latest singulation laser saws offer pulse repetition rates up to 120 kHz.
 Figure 4. The high power of the latest 532 nm lasers enables partial cutting (curves only) at competitive speeds. (Courtesy of Hanmi Semiconductor.) |
Why is such fast speed required? In principle, the laser could cut through an entire package, even at 1 mm total thickness, in a single cut. But the dwell time required to achieve this would cause far too much HAZ, i.e. a rough edge. So much better results are obtained by cutting using multiple sequential cuts. For a 1-mm thick package, about 10 such passes appears to be the optimum number of cuts to deliver an optimum combination of high throughput and edge quality. As a result of these factors, the state-of-the-art in singulation lasers are green DPSS lasers with higher output power (up to 38 watts), TEM00 output, pulsewidth of 60 ns, and repetition rates as high as 120 kHz.
Conclusion
Laser-based singulation offers advantages in terms of non-contact processing, no tool wear and no mechanical stress on the work piece. However, to be competitive, lasers must address all key considerations of singulation, including speed, cost, and edge roughness. Recent advances in green laser technology meet these requirements, and lasers appeared poised for wider adoption. AP
ANDREW CHANG, director of marketing, AP and inteconnects, and TIMOTHY EDWARDS, senior product manager may be contacted at Coherent, Inc. 5100 Patrick Henry Drive Santa Clara, CA 95054; 408/764-4342; Email: [email protected], [email protected].
The Short Story
Advances in short wavelength laser technology put laser singulation of IC packages, such as QFN and FBGAs, in a competitive position. This is particularly true for contoured products such as microSD cards, which require the high edge quality made possible with green laser technology.