Electronic packages (generally referred to as Chip Scale Package—CSP) are being produced in ever increasing numbers. Although the production of these packages is constantly growing, many problems remain unresolved regarding cutting quality and speed. In general, the underlying reasons are the different mechanical, thermal, optical and electrical properties of the materials involved. Today—thanks to a close collaboration between laser manufacturer Quantronix (Hauppauge, NY) and Synova SA (Lausanne, Switzerland), inventor of the Microjet—a revolutionary cutting system employing a laser beam guided within a water jet is opening new possibilities.
Electronic packages are generally made out of two main materials: the substrate to be soldered to the final circuit board and the mold compound that protects the chip itself and the connections of the chip with the substrate. The substrate is usually a copper lead frame or a thin glass fiber-enhanced polymer circuit board with solder balls. The mold compound is normally a brittle, black polymer (Bakelite) with different filler materials.
Economic, high-quality singulation of these packages is especially difficult in the case of copper and mold compound versions. This type of package is often produced using a matrix-array process to enhance productivity, but both the mold compound and the copper materials must be cut at the same time during the singulation process. The precise, burr-free singulation of copper is already an issue, so the cutting of the whole package is a real problem.
Standard CSP singulation methods today are mechanical sawing and conventional laser cutting. The problem with sawing is the inherent softness of the copper, leading to burr formation and the rapid wear of the saw blade due to smearing. Sometimes burr formation makes it impossible to properly solder the package to the final circuit board. Conventional laser cutting can lead to poor cut quality, as the thermal and optical properties of the materials used are different. Therefore, Quantronix and Synova propose an alternative method based on the process of water-jet guided laser cutting.
Cutting with laser and water
Figure 1. Laser Microjet operating principle.
With the unique Laser Microjet technology, a free laminar water-jet (see Figure 1) is used as an optical waveguide that guides a high-power laser onto the sample. The main advantages of this method compared to conventional laser cutting include: parallel sidewalls (even with thick mold compound layers), low thermal influence on the sample due to cooling occurring between the laser pulses exactly at the place where it was heated, efficient expulsion of the melted copper due to the high momentum of the water-jet and burr-free cuts and lower mechanical forces on the sample.
Optimal processing parameters for copper and mold compounds for a typical chip scale package. (Ipeak: peak intensity, Dnozzle: diameter of water-jet nozzle.)
A two-step process is applied for the singulation of chip packages. First the mold compound is cut with a 100-micron diameter nozzle at low laser peak intensity. In a second step the sample is inverted and the copper lead frame is cut with a 60-micron diameter nozzle at higher laser peak intensity.
The laser employed is the 532-CQE 100W green laser from Quantronix, chosen especially for its high average power (see Figure 2). It is a lamp-pumped Nd:YAG laser with intra-cavity frequency doubling. Cutting parameters and speeds for a package with a total thickness of 0.8 mm based on a 180µm-thick copper lead frame are presented in the table.
Figure 2. Laser performance at 20kHz pulse repetition rate.
The cut quality obtained is good for both copper and mold compounds especially compared to that achieved with mechanical sawing. Burr formation is almost absent at high water pressures, and the cuts are slightly tapered (see Figure 3). Whereas the mold compound can be cut at high speed (40 mm/s for a thickness of 800 microns), the cutting speed for the copper is limited but can be further improved.
Figure 3. Optical microscope images of the edge quality obtained with the parameters presented in the table: a) front side, b) backside.
The two principal strategies for increasing copper cutting speed are to increase the average laser power and to decrease thickness of the lead frame. Current research focuses on setting up a 200W green laser by doubling the laser light. In fact, it is expected that a linear relationship exists between average laser power and copper cutting speed as long as the peak power of the pulses stays constant and the pulses of the two lasers are alternated. In other words, using two combined lasers would double copper cutting speed. Therefore, speeds of up to 40 mm/s should be attainable.
The more cost efficient way to increase the copper cutting speed is to decrease the copper thickness in the cutting track. Today the majority of electronic packages are based on 200-micron thick copper lead frames, which is a reasonable thickness for laser-based package singulation. New developments tend to lean towards smaller packages, based on thinner substrates. Independently of total lead frame thickness, copper thickness can be decreased in the cutting tracks of etched lead frames with a minor design modification. This modification would lead to a nearly exponential increase of copper cutting speed.
In the future, both approaches of decreasing copper thickness in the streets and increasing average laser power could be combined for significantly higher electronic package singulation performance.
Laetitia Mayor is communications manager at Synova. She can be reached by e-mail at firstname.lastname@example.org.