The Many Uses for Lasers in Semiconductor Package Assembly Processes

By Ralf Schmidt, Rofin-Baasel

Lasers are used in a variety of applications within the semiconductor industry, including marking and micro applications such as decapping and scribing. Wafer singulation is one application where laser systems prove to be an efficient alternative to mechanical sawing (Figure 1). Cutting with a laser allows various geometries to be processed. Mechanical methods, on the other hand, are limited to cutting in only the x-y direction. The rounded edges of micro SD cards in mobile phones, for instance, are produced using laser singulation.

Figure 1: Laser systems are used to produce high quality cuts in wafer singulation applications. (Image courtesy of Rofin-Baasel)

Another interesting application is cutting quad-flat, no lead (QFN) packages. QFN refers to the way that the silicon chip is packaged within a flat resin block with no external leads. Contacts are on the base of the package, which is soldered directly onto a PCB without first drilling holes into it.

QFNs contain a moldable resin as well as a conductive compound such as copper. To separate these chips with a laser, a so-called hybrid process is used. Using ultra-short pulsed lasers,* the QFN can be cut via the copper site for electrical isolation, and the final separation of the IC packages will be done afterwards with a standard mechanical sawing system. The advantage of such process is that the speed of the mechanical sawing system will increase and the package quality can increase because of reduction of copper smear, solder bridging, etc.

In particular, the LED fabrication market needs tailor-made lasers for each different application requirement. Marking ID numbers directly onto the LED wafers is one common application, where unusual substrates, such as sapphire and other special coated wafers, can provide a challenge to the laser users. The housing of the finished LED product can also be marked for traceability, which is used in various industries.

Wafers are becoming thinner and thinner; some getting so thin that they can no longer be processed properly unless they are mounted on some other special carriers, such as a glass wafer. Lasers can be used to de-laminate the product wafer from this so-called transport wafer.

Scribing and cutting are important applications of laser use in the semiconductor and LED supply chain. In scribing processes, the laser cuts only around 30% of the thickness of the wafer material, relying on a subsequent mechanical process to break the wafer along the scribed lines.

Sapphire wafers are important for LED manufacturing and the applications, especially for high-brightness LEDs, are growing rapidly. Diamond or UV laser scribers are typical LED device singulation methods. However, the pressure to lower the LED manufacturing cost has boosted the demand for alternative dicing technologies that can increase throughput and lower the manufacturing cost of an LED chip dicing line.

Rofin has developed a process (patent pending) using their pico-second fiber laser in a unique way to scribe sapphire. It enables high processing quality and speed. Such technique is especially suitable for high-brightness LED's because of minimal brightness loss, which is a result of minimal edge scattering losses with minimal thermal effects. For example, on a 4" wafer with a thickness of 140µm, the pico-second fiber scribing process can deliver at least 50% more throughput compared to other methods, and up to 400µm thickness can be processed with a single pass. In addition to such a process advantage, the long lifetime of the pico second fiber laser along with their minimal servicing requirements reflects into the cost of ownership (CoO) in comparison to other laser technologies used.

*Rofin's Disc Laser technology

Ralf Schmidt is the International Sales Manager, Semiconductor Industry for Rofin, a global provider of lasers for industrial materials processing, including the semiconductor industry.