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12 SMT Magazine • March 2015 plied by mechanical fasteners. The bulk of this discussion is focused on mechanically-clamped conditions for TIM application. Development of well-performing TIMs must also include a set of conditions for storage, ship- ment, handling, assembly, and rework of the selected TIM. These conditions include specifi- cations, which vary depending on type of ma- terial, type of electronic module and assembly, and specifics particular to a given industry mar- ket segment, such as: • Shipping and storage temperature; humidity conditions by air, ship, truck • Dispensability (liquid-dispensable, paste format, die-cut preform, sheet form) • Manual placement • Reworkability (factory rework, field rework, or both) • Process requirements (if any) for mixing, pot life, dispensing, rework While TIM materials traditionally have rep- resented a very little cost in the bill of materi- als for a completed electronic system, the above list of operational requirements, as well as the handling, shipment, and rework requirements, presents a challenging prospect for new, well- performing material design at minimum cost. reliability and Failure modes Placing and clamping fasteners at the periph- ery of a module, such as a larger IGBT, can affect the gap at the interface between the IGBT mod- ule baseplate and the liquid cold plate to which it is attached. Baseplate thickness is an important factor in controlling such effects. During normal switching operation, these modules also exhibit mechanical expansion and contraction as operat- ing junction temperatures increase and decrease. The cyclical temperature changes can affect the metal baseplate of the module, which in turn has been demonstrated to cause a mechanical pump- ing action at the interface. This pumping action can affect certain types of organic TIMs, such as traditional silicone-based thermal greases. Pump-Out, Outgassing, and run-Out The mechanical pumping action experi- enced between two surfaces, such as described above with IGBT modules, contributes actively to loss of a TIM at the critical area in the inter- face. This phenomenon has been described in industry publications and thermal management conferences, and typically occurs with organic materials such as silicone-based thermal greases and gels and other paste forms of TIMs. Outgassing is a reliability concern when us- ing silicone oils as carriers, and other silicone- containing compounds. An early example of the impact of the outgassing issue, which is es- pecially important in certain industry segments such as optoelectronics, medical electronics, and space systems, is when silicone from ther- mal grease is outgassed and redeposited on opti- cal elements, such as lenses. An early internal specification, published by AT&T Technolo- gies (today, Alcatel Lucent), prohibited the use of silicone oil-containing compounds in com- pany manufacturing facilities. The first non- silicone-based thermal grease was introduced at that time. An increasing number of systems manufacturers continue to revise specifications to eliminate silicone compounds for reasons re- lated to the redeposition on optical and electri- cal interconnects. A major power semiconduc- tor manufacturer recently completed a major TIM testing and analysis program, eliminating the use of silicone-based TIM compounds for in- house use. Run-out refers to the tendency of certain types of organic compounds developed as a TIM to suffer material loss at the interface due to an insufficiently thixotropic compound for- mulation, especially at higher temperatures, in vertical mounting orientation, or when a com- bination of both occurs in an application. Or- ganic TIM formulations must be designed with the ability to remain within the interface, and a specified operating temperature range and stor- age (or non-operating) temperature range, in- cluding when mounted vertically. The above three types of reliability issues apply to organic compound formulations. Specifications developed by many system OEMs include weight loss testing under the range of designed operating temperatures for new assemblies in design, as a specific analytic tool to determine whether pump-out and run- out occur. Feature DEvELOPmEnTS WITh mETaLLIc ThErmaL InTErFacE maTErIaLS continues