Today’s advancements in packaging technology are constantly pushing the envelope of existing design parameters. Printed circuit board designers are integrating smaller and more functional devices into new designs. While the commercial market has driven the use of chip scale packaging technology, the defense sector is examining the issues concerning the use of these devices in military applications. The EMPF’s successes in the Micro-Electro-Mechanical Systems (MEMS), System on a Chip (SOC) and Integrated Power Systems (IPS) projects are definitive examples of bridging the technology gap between commercial and military applications while advancing the Navy’s technical capabilities.

The term chip scale may include configurations such as micro BGA, flip chip, chip on board and multi chip modules. Technically, the standard industry definition of a chip scale package is a package that is <1.2 times larger in total area than the area of the silicon die. Below we look at the impact that integrating CSP’s into SMT designs poses to current manufacturing processes.

Reflow Soldering
The reflow process for assemblies incorporating chip scale packages is similar to that of PCB assemblies using standard SMT components. There may be a need, however, to lower the volume of the convective air currents inside the reflow chamber when processing assemblies with extremely light weight components such as micro BGA and flip chip components, because it is possible to blow these components completely  off of their mounting locations during the reflow process.  When using chip on board (COB) components, the incorporation of wire bonding is necessary (Figure 5-1). The use of chip scale packages on an electronic assembly should be as straight forward as processing with standard SMT components. When the differences in manufacturing are understood and incorporated into the process, overall product yields should be compatible with assemblies made using standard SMT components. 

Material Dispensing
Dispensing of underfills and encapsulants requires more sophisticated equipment than may be required for production of assemblies using standard surface mount technology (SMT) components. The ability of the dispensing equipment to provide underside heating and the ability to program a various array of dispensing patterns is crucial to assembly processes when using chip scale components. Underside heating of the assembly is required in order to allow the capillary wicking action of the underfill compound to take place. When processing assemblies that incorporate COB components, the dispensing equipment must be capable of encapsulating the COB component without risking damage to the delicate wire bond connections between the component and the PCB substrate.

Inspection
By design, the electrical connections between the PCB substrate, micro BGAs and flip chip packages are hidden from visual inspection. The most reliable and comprehensive method of inspection of chip scale component connections is through the use of X-ray technology. By providing images based on material density, X-ray images show process defects such as solder bridging, open connections and solder voids within the solder connections.

Cleaning
In some cases, the standoff height of a flip chip component may be as little as 0.5 mils (0.0005”).  When using a no-clean chemistry in the cleaning process, standoff height may or may not become an issue. However, when using a process that requires assembly cleaning, the cleaning equipment must be able to penetrate these micro standoff distances to ensure effective residue removal underneath the chip scale component packages. This may require specialized cleaning equipment or modifications to the equipment and/or cleaning process currently in use.

Rework
When considering the rework process requirements with the use of chip scale components, certain product design consideration must be incorporated into the assembly. From an equipment perspective, the use of split vision for component alignment, the capability to regulate the dynamics of the thermal process, and the ability to provide sufficient underside and topside heating is paramount to the success of the rework process. Another consideration is the training of the rework operators. Differences in PCB and pattern designs, surface finishes, and thermal requirements of chip scale components  requires specialized operator training to avoid the potential of end product damage.

The Training Center offers a specialized three day course specific to chip scale manufacturing. If you would like additional information on this course, please contact the EMPF Helpline at 610-362-1320 or the registrar via email at registrar@empf.org.