OVERMOLDING PROVIDES COST, RELIABILITY AND MANUFACTURING IMPROVEMENTS
BY SCOTT D. BRANDENBURG
Since the introduction of the dual in-line package (DIP), semiconductor packaging has seen quite an evolution. As silicon semiconductor technology advanced, transformation of the IC package was necessary. IC packages with larger I/O capability were required to keep pace with the semiconductor industry. The move from through-hole technology (DIP, SiP) to surface mount technology (PLCC, QFP) satisfied this need for several years. As high-end microprocessor I/O increased to more than 200, array-type packages such as ball grid arrays (BGAs) were developed. More recently, 3-D packaging technology has led to multichip modules being overmolded in BGA-type packages. While overmolding single ICs has been done for decades, this article explores development of a packaging concept in which overmold technology is used to encapsulate the entire circuit board assembly to form the electronic enclosure.
Concept Description
Traditional product encasement methods consist of a circuit board assembly located between two metal halves that are held together with screws (clamshell design). A peripheral seal generally is applied between the case halves to seal the unit from moisture. Heat sinking the IC components is commonly accomplished by a heat rail attached to the circuit board or by direct contact to the product case. Assembly of these packaging methods can be cumbersome.
The operating environment of the automotive controller is becoming more severe. Auto engine control units are seeing rising vibration and ambient temperature levels. In a traditional metal case, PCB support becomes an issue and component interconnections may be susceptible to vibration damage. Silicon integration can lead to higher power densities, increased device temperatures and potentially decreased reliability. Newer technology packaging solutions may be necessary to address these constraints, while also meeting the ever-growing cost pressures from global competition.
With the use of innovative packaging techniques and recent advances in epoxy thermoset materials, transfer molding can be used to form a plastic package to encase the entire product electronic assembly. The application of flip chip and µBGA packages has enabled the form factor of the automotive electronic controller to be reduced by as much as 75 percent. With this reduction in package size, it becomes economically feasible to use thermoset epoxy to overmold the entire circuit board assembly.
 Figure 1. Exploded view of overmold. |
The overmold electronic packaging concept consists of a PCB assembly populated with flip chips and other surface mount components adhesively mounted to a metal backplate, which is then overmolded to form the electronic enclosure. One of the biggest challenges was to develop methods to incorporate a product connector header in the molded module. This was accomplished by insert-molding a modified product connector (Figures 1, 2).
 Figure 2. Overmold controller. |
The mold compound surrounds the PCB assembly and the top surface of the metal heat sink. The combination of proper PCB thickness and the use of backplate support pedestals create a rigid structure that can withstand the molding process. Some design enhancements to the PCB were also necessary so that the assembly could be overmolded. Mold flow modeling can be used to help designers optimize component placement and circuit board layout (Figure 3).
Material Development
Thermoset materials have been used in the electronics industry for decades, because of their suitable properties. Retention of electrical, mechanical, thermal and chemical properties at elevated temperatures and high moisture resistance make them useful for packaging electronic components. Thermoset epoxies are highly filled, possess low shrinkage and have a low susceptibility to stress formation. They offer good adhesion to most materials, high tensile and vibration strength, high heat resistance and high chemical attack resistance. These properties make thermoset epoxy a suitable material to overmold PCB assemblies.
Some improvements in thermoset epoxy materials were necessary so that a relatively large assembly could be overmolded. Lower viscosity resins with optimum gelation times were required so that the large cavity could fill prior gelation. Viscosity increases greatly after gelation. Therefore, if gelation were to occur prior to filling, a large shear stress may be placed on the components. This could disturb the solder joints. For high-volume manufacturing processes, a relatively fast cycle time is desired. A compromise on gel, spiral flow and cure time is required. The material properties must be carefully chosen so that an optimized manufacturing process can be developed.
 Figure 3. Simulation of molding. |
Many epoxy materials require a long post-mold cure at elevated temperatures to obtain a high glass transition temperature. Large batch cure ovens with a 4-hour post mold cure generally are not part of a lean manufacturing process. A material that obtains a glass transition temperature that exceeds the maximum operating temperature without postmold curing is a good candidate. The relatively large size of the overmolded packages makes a material with low shrinkage an absolute necessity. The backplate and mold compound thickness should be designed so that the CTE mismatch between the two does not cause warpage or detrimental bending of the module during temperature excursions.
When overmolding an entire electronic assembly, several dissimilar materials are encapsulated. The material properties of the PCB, metal backplate, silicon flip chips and other surface mount components have to be taken into account. Properties of the thermoset epoxy are chosen to prevent inadvertent interactions with any of the materials being molded. The thermoset material properties were optimized, with the most important of these being CTE and modulus. The selected material was an adequate match to the backplate and PCB assembly, because no delamination occurred after temperature cycling.
To underfill small-pitch flip chip devices, a material with a small spherical filler size is desirable. The small size is necessary so that the filler particles (silica spheres) flow between the bumps of small-pitch flip chip devices. A larger particle size could block the flow of compound underneath the die, creating voids or allow only the resin to flow under the die, which would likely result in lower flip chip bump reliability.
The thermoset epoxy compound adheres well both chemically and mechanically to the epoxy glass laminate structure. Inspection of overmolded samples shows that the thermoset epoxy is embedded in the epoxy glass of the PCB to the extent that the PCB is ripped apart during destructive physical analysis. Proper adhesion of the mold compound to the PCB and the components is imperative for a reliable product. Once adhesion is lost between two surfaces, microdelamination can occur, which may propagate to a nearby solder joint and create a solder joint fracture. Choosing the correct thermoset material is necessary for producing a reliable and durable product.
Process Development
Transfer molding has been around for decades. A traditional transfer mold press is at the heart of any molding manufacturing process. Transfer molding generally is a fast, consistent manufacturing technique that results in high-quality parts at high yield and throughput. This relatively simple process can be highly automated, which makes it a suitable choice for a lean manufacturing line. Due to the relatively large size of most engine controller packages, only a maximum of four parts per shot can be achieved. This may seem small compared to molding a strip of IC packages, but throughput is still good, considering that the underfill process and product encasement is being completed on four modules in about 2 minutes.
Several thermoset materials were evaluated during the development phase. Once a material set is finalized, the molding profile can then be optimized. Many of these parameters are different from those than are used in traditional molding processes. Since eutectic solder is used to attach components to the PCB, the molding temperature is lower than used in traditional molding to prevent solder movement/reflow during the molding process. The molding pressure is also set slightly lower than seen in standard processes. Considering the relatively large size of the overmolded assembly, transfer times were similar to those of smaller IC packages.
The mold cavity design, thermoset material and mold press settings are all interrelated. Properly sizing and locating the gate and vents in the mold chase in relation to the PCB assembly is also important. Vacuum-assisted molding may be helpful in achieving proper flip chip underfill. It is best to design the PCB, enclosure and mold cavity in conjunction with each other so that an optimized process will be easier to develop. Experimentation has demonstrated that when molding different product enclosure designs, the molding parameters need to be readjusted. But once a design is finalized and a material set is chosen, the process window can be set wide enough to result in a robust manufacturing process.
Advantages of Overmolded Packaging
Several advantages can be realized by using this packaging method. The use of thermoset epoxy to encapsulate the assembly provides good mechanical and moisture protection, as well as vibration resistance.
Since the thermal conductivity of thermoset mold compound is relatively low (0.7 W/m °K), an overmolded circuit board assembly may be limited to low-power applications. Therefore, an integral heat sink (metal backplate) is included in the overmolded packaging concept. Flip chips and other surface mount components that require additional heat sinking are bonded to the metal pedestals with thermally conductive compounds.
Using this concept, flip chip thermal management is improved by nearly 100 times compared to a flip chip on a PCB in still air. Since the mold compound is 20 times more thermally conductive than air, this concept enables significantly improved overall package heat dissipation than traditional electronic packaging, which uses air inside the case. The use of the integral heat sink also allows this packaging concept to be used in relatively high-power density applications operating in harsh environments (Figure 4).
 Figure 4. Thermal resistance vs. die size. |
An advantage in the manufacturing process is realized if the flip chips can be underfilled simultaneously during the overmolding process. Since the overmold process can replace many assembly steps, the manufacturing process can be simplified by using this 1-step process. Capillary underfill machines and long conveyer furnaces to cure underfill material can be removed from the manufacturing floor. Using the transfer molding process for underfilling the flip chip devices increases manufacturing throughput. The traditional capillary underfill process is relatively slow and is performed one chip at a time. Some electronic controllers may include 10 to 15 chips. When underfilling during the molding process, all of the flip chips can be underfilled at one time and cured in just 2 minutes.
Because underfilling occurs in seconds during the overmolding process, silica filler particles do not have time to segregate, resulting in uniform particle distribution under the flip chip. This translates to homogeneous material properties, which results in a lower, more uniform stress under the flip chip and better interconnection reliability. Since the flip chips are underfilled with pressure (transfer molding), complete underfill occurs even though the mold compound has much higher filler content than traditional capillary underfill. The higher filler content enables a lower CTE underfill, again resulting in higher solder bump reliability.
Finally, several other manual assembly steps related to casing the module can be removed from the manufacturing process by overmolding the product in plastic. If materials properties of the system components are carefully chosen, product reliability can increase by nearly two times. Using a transfer mold process to form the enclosure can result in a cost-effective electronic packaging solution.
Conclusion
With modifications to the PCB assembly, design of a heat-sinking backplate and the use of new epoxy mold compounds, an overmolded electronic product enclosure can be formed using a conventional transfer mold press. Simultaneously underfilling the flip chips and overmolding the electronic assembly can result in significant improvements in the manufacturing process. Cost, reliability and manufacturing improvements can be realized over traditional electronic packaging designs by packaging the assembly using the overmolding process.
References
- Wright, R.E., “Molded Thermosets — A Handbook for Plastics Engineers, Molders, and Designers,” Hanser Publishers, 1991, ISBN 3-446-15820-0.
- Miles, T.R., Rector, L.P., Gong, A, Lobinanco, T., “Transfer Molding Encapsulation of Flip Chip Array Packages: Technical Developments in Material Design,” 3rd Annual Semiconductor Packaging Symposium/Session III, SEMICON West 2000, pp. E-1 to E-6.
- Gilleo, K., Chen, T., Cotterman, B., “Molded Underfill for Flip Chip Package,” High Density Interconnect, June 2000, pp. 201-205.
Acknowledgement
The author would like to thank Dr. J. Stephen Tsai, Tom Degenkolb, Dave Laudick and Jeff Daanen for their help to develop the technology of this packaging concept. Thanks also to Rick Tubbs for providing metallurgical analysis.
SCOTT D. BRANDENBURG, senior staff engineer, may be contacted at Delphi Electronics and Safety, One Corporate Center M/S D16, PO Box 9005, Kokomo, IN 46904-9005; (765) 451-3060; e-mail: [email protected].