When referring to "chip scale" component packages, it is still unclear exactly what constitutes a chip scale device. Does the term chip scale include microBGA packages as well as 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. For the purposes of this article, which is intended to discuss the various manufacturing techniques and processes involved in using chip scale packages, we will include processing methods for microBGA's, and direct chip attach technologies.

Printed Wiring Board (PWB) Design Considerations
Successful processing of PCBs using chip scale component packages begins with the design of the PWB substrate. Issues such as circuit routing, land pattern and via design, solder mask and surface finish, all play an integral role in the overall success of the assembly process. Routing of circuit traces and associated vias increases in complexity when using area array devices. In order to compensate for the additional routing complexities, additional substrate layers may be required. The selection of land pattern design and solder mask placement is critical to solder joint integrity, reliability and verification. The use of non-solder mask defined land areas imparts less solder joint stress than does the use of solder mask defined lands. Providing a solder mask dam between the land area and any associated vias will prevent solder scavenging or solder flow into the vias during the reflow process. An additional concern is the PCB surface finish. When using microBGA and chip scale component packages, the fine pitch of the components usually requires coplanarity of the component land areas. The use of a standard HASL (hot air solder level) tin/lead surface finish will not provide the required coplanarity for these devices. Alternative surface finishes such as OSP (organic solder preservative), immersion silver or electroless nickel/gold (ENIG) may be required. Violation of these coplanarity issues will inevitably lead to open solder joint connections on the final product assembly.

General Assembly Considerations
The incorporation of chip scale devices into circuit designs has become increasingly necessary in order to fulfill the industry requirements of more end product capability and performance into a smaller overall package design. Some of the process considerations that must be addressed in order to successfully manufacture products using chip scale packages include material dispensing (primarily for underfilling and encapsulation), inspection, cleaning and rework. The characteristics of chip scale components may require modifications to these assembly processes in order to compensate for decreased component package dimensions.

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. Having the underfill material completely fill the entire area under the microBGA or flip chip is crucial for overcoming coefficeint of thermal expansion (CTE) mismatch between the PCB substrate and the chip scale component as well as for adding enhanced mechanical strength to the component. When processing assemblies that incorporate chip on board (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, microBGA's and flip chip packages are hidden from visual inspection. There is some visual inspection equipment that allows visualization of the underside of the component package. However, in most cases, the equipment is limited by the inability to fully examine the entire package. In these instances, process anomalies such as solder bridges, open connections and voids may not always be detectable. 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 will show process defects such as solder bridging, open connections and solder voids within the solder connections. Although comprehensive, X-ray imaging requires special operator training in order to detect some of these anomalies that may appear as subtle changes in the image.

Cleaning
Smaller chip scale components have a considerable difference in component standoff height from the PCB substrate than do standard SMT components. 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 your 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. Was the assembly designed with sufficient clearance around adjacent components (keep-out distance) to allow for rework tooling? Is the PCB substrate surface finish and land pattern design able to withstand the rigors of the rework processes? If underfill material is required, is it reworkable?

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 design, surface finish and thermal requirements of chip scale components compared to standard SMT components requires specialized operator training to avoid the potential of end product damage.

Stencil Printing
When dealing with chip scale packages, the standard process of stencil printing solder paste onto the PCB substrate using a standard 6-mil thick stencil may not necessarily apply. In most instances, when using flip chip and microBGA components, tacky flux is used in place of solder paste, and the deposited material is approximately 2 to 4-mils in height. In some cases, depending on the component pitch, microBGA components can be placed into solder paste, however the standard height of the deposit will be less than the usual 6-mils customarily used on standard SMT components. How does this affect the stencil printing process? Primarily, the process will remain the same but major design changes to the standard 6-mil stainless steel stencil will be required. When dealing with ultra fine pitch components, a chemically etched stencil will not yield the precision required; therefore, a laser etching or electroforming stencil manufacturing process will be necessary. This will increase the cost of the stencil. On PCB assemblies that incorporate both standard SMT and chip scale packages, step etching the stencil to provide for smaller material deposits on the ultra fine pitch component lands will be necessary. This means a more flexible squeegee blade, usually rubber or urethane, will be needed. Depending on the pitch of the components used on the assembly, it may be necessary to deviate from the standard mesh #3 solder paste and use a mesh size of #5 or #6. The fine mesh solder paste allows for better paste transfer through the stencil apertures and better release from the stencil apertures in fine pitch applications.

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 microBGA and flip chip components. It is possible to physically blow these components off of their mounting locations during the reflow process. If it is necessary to lower the convective air currents in the process, thermal profiling must be performed to verify the thermal dynamics of the process are viable.

Summary
There are differences in process parameters that must be considered when processing electronic assemblies using chip scale packages. When these differences are understood and incorporated into the manufacturing processes, overall product yields should be compatible with assemblies using standard SMT components; however, when using COB components, the incorporation of wire bonding is necessary. The use of chip scale packages on an electronic assembly should be as straight forward as processing with standard SMT components.

ACI offers a specialized 3-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 via email at helpline@empf.org.