Keith Sweatman, Takatoshi Nishimura, Kenichiro Sugimoto, Akira Kita
Nihon Superior Co., Ltd.
While a significant level of voiding can be tolerated in solder joints where electrical conductivity is the main requirement, voiding at any level severely compromises thermal conductivity. For example, in Light Emitting Diode (LED) lighting modules effective conduction of heat through the 1st level die attach to the substrate and then through the 2nd level attach to the heat sink is critical to performance so that voiding in the solder joints at both levels must be minimized. Voids in solder joints are the result of bubbles of gas that do not escape before the solder has solidified.
While there is the possibility that air can be entrapped in the bond area during the reflow of solder paste the gases in the bubbles are generally considered to come mainly from the flux medium, by volatilization of solvents, as by-products of activator reactions with metal oxides, and from decomposition of resins and other constituents. Whether these gases escape from the solder joint or remain as voids depends on many factors including joint area and geometry.
Since areas of non-wetted substrate provide points of attachment for bubbles the solderability of the substrates and the activity of the flux are other factors that affect the incidence of voiding in the solder joint. Volatiles released during the time when the solder powder particles are melting and coalescing are the main source of bubbles so that the shape of the reflow profile can have a major effect on the incidence of voiding.
In this paper, the authors will review the factors that influence the incidence of voids in small and large area solder joints that simulate, respectively, the 1st and 2nd level joints in LED modules and discuss mitigation strategies appropriate to each level. They will also report the results of a study on the effect on the incidence of voids of flux medium formulation and the optimization of the thermal profile to ensure that most of the volatiles are released early in the reflow process.
While it is not possible to completely eliminate voids they can be minimized by a systematic approach based on recognition of the factors involved in the generation of voiding and their mitigation. In the study reported in this paper, the beneficial effects of the following factors were demonstrated with target voiding levels being achieved.
- modifying the formulation to minimize the release of volatiles during the part of the reflow profile when the solder is molten
- optimizing the formulation for reducing the surface tension of the solder
- maximizing the chance of escape from the molten solder by prolonged TAL
- vacuum treatment while the solder is molten
Incidentally, the benefit of a nitrogen atmosphere was confirmed.
Initially Published in the IPC Proceedings