SMT Magazine

SMT-Apr2017

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68 SMT Magazine • April 2017 Intermetallics Intermetallic compounds form when two unlike metals diffuse into one another creating species materials which are combinations of the two materials. Intermetallic growth is the re- sult of the diffusion of one material into anoth- er via crystal vacancies made available by de- fects, contamination, impurities, grain bound- aries and mechanical stress. There are a num- ber of locations within the electronic package where these dissimilar metals are joined. These include die level interconnects and wire bonds, plating finishes on lead frames, solder joints, flip chip interconnects, etc. Growth of interme- tallics during the storage period can occur and reduce the strength. Intermetallic growth rate is strongly tem- perature-dependent and doubles for each 10°C temperature increase. This aging process can be slowed by appropriate cooling. However, the risk of whisker formation of tin alloys increases with decreasing temperature. Studies and prac- tice have shown that a storage temperature of 12°C is optimal in order to best mitigate both risks. SMT Rich Heimsch is a director at Protean Inbound and Super Dry-Totech EU in the Americas. To read past columns or to contact Heimsch, click here. midity protection (a.k.a. moisture barrier) bag, reliably and effectively protect from moisture dif- fusion. For storage periods of more than five years, a combination of the two systems is recom- mended. The storage in humidity protection bags with nitrogen present within a simple dry storage cabinet with 5% RH. Critical to the effectiveness of the bags, however, is that the construction is mechan- ically stable and exhibits a very low percent- age of diffusion. The IPC/JEDEC-STD033C de- mands a Moisture Vapor Transmission Rate (MVTR) of less than 0.002 g/100 in² in 24 hours at 40°C. This demand is only met by bags which have a thickness of 150 μm; 90 μm-bags have a substantially higher diffu - sion percentage and are therefore not suitable. Moisture barrier bags are available that remain significantly below the maximum value as laid down in the IPC standard with an MVTR of 0.0006 g/100 in². The bags must of course also be ESD-safe; they must be marked as receptive to humidity and be provided with a label upon which the moisture-sensitivity-level and the packing date are clear. Procedures such as those outlined above have been successfully utilized to eliminate the oxidation and diffusion hazards of long term storage. However, another risk of long term storage must also be considered. SOLUTIONS FOR LONG-TERM STORAGE OF ELECTRONIC COMPONENTS AND COMPOSITIONS With RoHS requirements in full swing, com- panies must adapt and adapt quickly as lead based soldering will be completely phased out by 2019 in Europe. In this interview during the recent IPC APEX EXPO 2017 event in San Di- ego, California, AIM Solder's Timothy O'Neill, technical marketing manager, talks about their two new lead-free alloys that are proving to enhance reliability for high end applications while minimizing issues with voiding. Watch the interview here. Real Time with... IPC: Tim O'Neill Discusses Latest Developments in Lead-Free Soldering Market

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