The Elimination of Whiskers from Electroplated Tin
When RoHS lead-free regulations began to take hold globally, tin and its alloys were the first choice as alternatives to eutectic tin/lead. On the solder side, the transition to using these alternatives has moved forward, but on the surface finish side, replacing tin/lead has posed greater challenges. This paper discusses two approaches to dissipating the internal stresses in and external stresses on the deposit that are known to initiate whisker formation.
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By Masanobu Tsujimoto, Shigeo Hashimoto, Masayuki Kiso, Raihei Ikumoto, Toshikazu Kano and Genki Kanamori, C. Uyemura & Co. Ltd. (Japan); and Don Gudeczauskas and George Milad, C. Uyemura & Co. Ltd. (USA)
As RoHS lead-free regulations began to take hold globally, tin and its alloys were the first choice as an alternative to eutectic tin/lead. On the solder side, the transition has moved forward and solutions have been implemented, such as the Sn-Ag-Cu (SAC) family of lead-free (LF) solders, for paste reflow and tin/copper for hot air solder leveling (HASL). The industry is constantly making progress adapting its materials and processes to the higher reflow temperature profile for these LF solders. Today there is a much better understanding of the nature of the intermetallic (IMC) bond as well as the reliability of LF solder joint.
On the surface finish side, replacing tin/lead has posed greater challenges. Component leads and connector finishes were being converted to tin as an obvious alternative. This works well as a soldering surface. However, any part of the lead or the connection surface that is not soldered to has shown a tendency to form tin whiskers over the life of the part. Internal stresses in the deposit due to IMC formation or external stresses on the deposit are known to initiate whisker formation.
In this paper two approaches are implemented to dissipate the stress that is formed. The first is to modify the substrate surface to control the growth in thickness and direction of propagation of the IMC. The second is to modify the large columnar tin deposit crystal structure to mimic the fine equiaxed structure of tin/lead solder. The former is achieved thru controlled micro-roughening of the substrate and the latter by the use of additives to the plating bath. Data will be presented to show that by implementing these two modifications, the stress causing tin whiskers is dissipated and tin whisker formation is inhibited.
Keywords: tin whisker inhibition, tin/lead solder replacement
Electroplated pure tin and tin-based alloys are being used as alternatives to tin/lead in the majority of electronic components. These alternatives are known to produce tin whiskers resulting in short circuits on these components.
In the case of a tin finish on copper and copper-based alloys, the major cause of tin whisker formation is compressive stress. The stress is mainly caused by irregular growth of a copper-tin intermetallic compound (IMC) at ambient conditions.
It is known that tin whiskers are readily formed on electroplated tin deposits on copper and are not observed on electroplated tin/lead deposits. The crystal structures of tin and tin-lead deposits are different. The crystal structure has a direct impact on tin whisker formation.
A tin deposit with a modified crystal structure (similar to tin-lead deposits) is capable of preventing whisker formation by dissipating and delocalizing the stress that cause whiskers.
As shown in Fig. 1, stress, channeled along the boundaries of the large grained columnar tin deposit, is responsible for the emergence of tin whiskers. Stress may be internal or external (Fig. 2). The primary source of internal stress is attributed to the non-uniform increase in the thickness of the IMC layer over time at ambient conditions (30°C, 60%RH for 4000 hr). Another condition that produces internal stress is exposure to high temperature and high humidity (55°C, 85%RH for 4,000 hr) for extended periods of time, which gives rise to oxidation and/or corrosion. Internal stress could also be induced by thermal cycling (-55°C to 85°C, 1500 cycles) due to mismatched coefficients of thermal expansion. The latter two forms are commonly used to induce internal stress in controlled experiments. External stress is also known to initiate whisker growth. An example is the stress induced by press fit connectors.