Honeywell

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Improving Solder Joint Reliability in SiPs Using Plasma-Based Nanocoating for Top Coat
Keywords: SIP, Nanocoating, Solder
With the industry turning to heterogeneous integration as the new path forward to achieve power, size, performance and cost requirements of mobile computing devices, system-in- package (SiP) technologies are now among the most popular packaging configurations, as they allow for the integration of more than just multiple die into a single package. Hindered by the smaller size of wearble devices, many companies are migrating to a System in Package design (SiP) to replace the Printed Circuit Board used in devices. For example, the application processor, support processors, sensors, storage and memory for wearble devices could be rolled together in the one SiP module that takes up less space than a PCB. The SIP can be extended to also include a variety of microelectromechanical systems, as well as digital and analog ICs and passives assembled into modules to deliver increased functionality in a small form factor. SiP modules are subsequently mounted onto printed circuit board (PCBs) and stored, to be later integrated into mobile computing devices such as smartphones, tablets, notebooks and wearable devices. Above all, devices are driving towards higher and higher data transfer speed. This is the impetus for developing more intricate 2.5D and 3D packaging schemes. For example, in 2.5D packaging the connectivity is provided using Si interposers with through vias. In contrast, in 3D schemes, the dies, containing vias, are stacked on top of each other. In both schemes, the entire package is approximately less than 50 micron. Furthermore, as with before, the dies require redistribution layers and underfills which are pliant dielectric materials. All these materials must have various properties including mechanical, thermal, electrical and chemical properties which are conducive to small size packaging for SiPs. Plasma polymerization is a known technique used for producing polymer thin films with specific properties, which can be adjusted to meet the exacting demand of the packaging schemes cited above. Because of the flexibility of plasma polymers, it is possible to create packages with greater than 300 I/O counts with enough stress buffering during temperature cycle testing. Plasma polymer thin films can be readily deposited for temporarily bonding thin wafers to carrier wafers for processing. This may eliminate several steps in the process. It is possible to completely supplant the molding compounds for packaging materials with plasma deposited thin film polymers to help prevent die shift in packaging. During storage, fully populated circuit boards are subject to environmental stresses that affect their reliability and lifetime, as well as the lifetime of all SiPs they support. To maintain the solder- joint reliability of SiP modules, as well as to ensure reliability once they are integrated into devices, top coats are applied to the boards. Traditional methods use parylene-based polymer conformal coatings that, while not waterproof, shield SiP modules against environmental conditions like moisture, chemicals, dust and debris, as well as insulate sharp edges and solder joints. However, these approaches are similar to applying a coat of paint, and are not reworkable. As such, they are detrimental to reliability, especially on the solder joint level, because the solder joints are likely to wear down after reliability testing. In addition to environmental protection, the coating must possess dielectric properties consistent with the high frequency circuits used in mobile devices. The dielectric and electrical properties of thin film materials as measured over a wide range of frequencies are of considerable interest within the electronics industry. Interest in the dielectric properties of coating materials (and products) has been principally for predicting dielectric strength, i.e., breakdown voltage, and heating rates describing the behavior of polymeric coating materials when subjected to high frequency or microwave electric fields in high-speed telecommunication applications. Semblant’s plasma nanocoated polymers is among the lowest loss polymeric metrials in the frequency range of interst in the telecoomunication industry. This coupled with the high dielectric constant allows the coating can withstand voltages up to 500V before breaking down. This presentation describes an innovative chemical nano-coating material that is engineered not just to PCBs, but SiP packages as well as the chips and components they contain. It will discuss a plasma coat process that is 100% reworkable, and that surpasses traditional conformal coating processes in protection and performance. Lastly, it will present the results of a recent collaborative effort between Semblant, Amkor, Celestica and Universal Instruments, in which Semblant’s PlasmaShield 400 was evaluated in terms of solder joint reliability performance.
Simon McElrea,
Semblant
Scotts Valley, CA
United States


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