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FOUR WAYS TO REDUCE VOIDS IN BGA / CSP PACKAGE TO SUBSTRATE CONNECTIONS

Voids in BGA/CSP package to substrate connections can cause issues with thermal management, drop shock resistance, and signal interference. Zero voids are always the best solution, but this is not always easy to achieve. Solder paste printing, reflow, alloy and flux chemistry all have significant influences on the amount of voiding one can expect in an assembly. This paper will discuss how print volume (stencil design), peak reflow temperature, paste flux chemistry and alloy selection affect the level of voiding to be expected when BGA/CSP packages are assembled in a lead free process. INTRODUCTION Voids in solder joints are considered one of the more detrimental defects in electronic assembly as area array devices have now evolved to a point where mass production of fine pitch devices with increasing functionality are common. The factors affecting void formation are complex and involve the interaction of many factors. In area array components such as BGA (Ball Grid Array), PBGA (Plastic Ball Grid Array) or CSP (Chip Scale Packages), solder joint reliability between the component and the board is one of the most critical factors as finer pitches and smaller component solder sphere volumes reduce the amount of solder in the corresponding solder joint. Voids have shown to reduce mechanical robustness of the board level interconnection and consequently affect the reliability and the conducting performance of the solder joint. The effect of voids on the reliability of a solder joint depends on size, frequency and location [1]. Voids are defined as cavities formed in the solder joint. Voids are largely caused by the amount of outgassing flux that gets entrapped in the solder during reflow [2]. Voids are essentially gas bubbles that have a much lower density than molten solder. The buoyancy of these regions causes the bubbles to rise to the top of the solder joint [3]. Other sources of voids can be via in pad. The outgassing flux is typically produced by the evaporation and thermal decomposition of flux constituents getting trapped in a solder joint during reflow. Reactions of chemical constituents in the flux and substrate at elevated or solder reflow temperatures are also a common cause for gas evolution. Flux is typically a complicated homogeneous solvent based mixture with many organic additives. Being organic in nature the boiling points and/or thermal decomposition temperatures of many constituents are less than 250C. Other outgassing sources may be substances generated from or reactions with the substrate, component metalization, or the solder powder or sphere surfaces. The location and the size of the voids are perhaps the key factors that influence the effect of the voids on the performance and reliability of solder joints. It has been an industry observation that voids in solder joints tend to accumulate towards the top of the solder joint (the interface between the package and the solder joint). Size is perhaps the most critical factor negatively impacting the solder joint by becoming a source of entrapment, a stress riser, thermal barrier and electrical degradation by restricting the path of current flow. Larger voids typically reduce the robustness of the solder joint compared to small voids because the incidence of failure increases dramatically as the solder material thickness between the void and the ball exterior decreases. For example a 0.007” ball with a 50% void has approximately 0.0017” on each side of the void, while a 0.004” ball with a 50% void has only 0.001” on each side. The voids impact becomes more severe if it is near one of the bonding interfaces. Void location may be equally important as reduced cross sectional area near the bonding interfaces can adversely affect reliability. A reduction in cross sectional area, specifically when voids are located at the ball/interface attach site, current flow is restricted, shear strength is reduced proportionally to the decrease in bonded material and the solder joint stability will degrade more rapidly with increased temperature cycling. Voids near the interfaces with the greatest CTE (Coefficent of Thermal Expansion) will degrade and fail in the shortest time. A solder joint in an electronic assembly serves electrical, mechanical and thermal functions, the most important being the conduction of electrical signals. Inherently, the resistance of the solder joint should be as low as possible and there should not be much deviation in resistance between adjacent solder joints in order to achieve uniform conductivity. It has been observed in industry that the resistance of the solder joint will increase with the occurrence of large or many voids as the cross sectional area of the solder joint is considerably reduced. The mechanical function of the solder joint is to provide the connection and support to the component. The electronic component is continuously affected by the stress and strain as a result of the mismatch in the CTE, and the solder joint should be able to withstand all the stress and strain imposed on the component. An active component in use can produce a significant amount of heat during its operation, and this heat needs to be dissipated to prevent overheating or cause the breakdown of the component. The conductive heat transfer through a solder joint can be modeled based on Fourier’s law: