SMT007 Magazine

SMT-Feb2015

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58 SMT Magazine • February 2015 cern over, known for ages but limited by lead addition, phenomenon: tin whiskers. This phe- nomenon can significantly influence the reli- ability of solder connection and as a result in- fluences the work of the entire PCB. Tin whiskers are the crystals growing from the tin or tin alloy surface which can be dan- gerous for circuit reliability because of electrical current leakage and shorting due to bridging of adjacent conductors, metal vapor arcing (plas- ma) at low pressure, increased electromagnetic radiation, and damage from device littering by debris and contamination [1, 2] . The growth of tin whisker is caused by the compressive stress re- laxation [3] . The copper intermetallic compounds and surface layer of oxygen can strongly influ- ence whisker growth. This paper presents the results of energy-dispersive X-ray spectroscopy (EDS) studies showing the relationship between tin whisker formation and copper and oxygen surface content in whisker neighborhood. experimental To estimate the dependence of tin whisker growth on copper and oxygen surface con- tent, the test samples were prepared using PCB technology. In the experiment, the glass-ep- oxy laminates with a Cu foil of 17 μm thick- ness were used. One part of the samples was prepared with an electroless Ni immersion Au sub-layer, using a Pd activator. The Ni thickness varied from 4–6 μm and the plated Au layer was about 0.05 μm. The conductive pads were covered with Sn100 and Sn99.3Cu0.7Ni solder alloy. The alloys were applied by hand solder- ing with water flux application. The soldering temperature was dependent on the alloy com- position. The thickness of all Sn layers was in the range from 10–20 μm. The samples were subjected to long-term exposure at temperature of 60°C and relative humidity 87% RH for 3000 h. Scanning electron microscope (SEM) images and EDS were performed with a Philips XL30 microscope. To determine the local content of elements—in particular copper and oxygen— X-ray spectra produced under the influence of the incident electron beam were performed in a scanning electron microscope. EDS analysis was performed for several points of the specimen: on the whisker body and on the whisker base. For the comparison purpose the same analy- sis was performed in the random places far from the nearest whisker, in the middle at the edge of solder pad. The electron beam energy, used for EDS analysis, was at the level of 10 keV, so the penetration range of electrons in tin calculated according to the Kanaya-Okayama model for the 10 keV is 0.65 μm [4] . results For the samples exposed for 3000 h in 87% RH and 60°C whiskers were found on Figure1: SeM images of the samples soldered directly on Cu layer after 3000 h exposure at 87% rH and 60°C—the edge of the solder area. (A) Sn100, (b) Sn99.3Cu0.7ni. Feature TIN WHISKer GrOWTH ON THe SurFaCe OF TIN-rICH Lead-Free aLLOyS continues

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