The Elimination of Whiskers from Electroplated Tin
The introduction of lead free (LF) solders, have come to the forefront as the primary replacement for tin-lead. On the surface finish side, plated tin/lead has posed challenges. Here, by modification of the substrate surface and deposit structure, tin whisker formation is limited.
Masanobu Tsujimoto, Shigeo Hashimoto, Masayuki Kiso,
Raihei Ikumoto,Toshikazu Kano and Genki Kanamori
C. Uyemura & Co., Ltd. - Hirakata Japan
Don Gudeczauskas and Al Gruenwald
Uyemura USA - Southington CT
Editor’s Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2016 in Las Vegas, Nevada on June 6, 2016. A printable PDF version of the full paper is available by clicking HERE.
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, like the Sn-Ag-Cu (SAC) family of lead-free (LF) solders, for paste reflow and tin/copper for HASL (hot air solder leveling). 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 potential 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 by controlled micro-roughening of the substrate and the latter by the use of additives to the plating bath. Data is presented showing that by implementing these two modifications that the stress causing tin whiskers is dissipated and tin whisker formation is inhibited.
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 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 tin deposit and tin-lead deposit differ in crystal structure. 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 causes whiskers.