Whisker formation on matte Sn influencing of high humidity

Abstract

With the 1 st of July 2006 as deadline for the European RoHS directive approaching fast, the need for conversion of SnPb plated components to Pb-free alternative is increasing. Most component suppliers are looking at pure Sn plated or Sn-alloys electroplating as an alternative for the SnPb plating. However, electroplated Sn and Sn-alloys are associated with spontaneous whisker growth.In these past 4 years the E4 (Freescale Semiconductor, Infineon, STMicroelectronics and Philips) have extensively studied matte Sn in relation to whisker formation, exceeding the most common whisker assessment tests in considered time for bare parts, parts soldered on board and parts exposed to simulated reflow.The purpose of this study was to identify the main causes for whisker growth and to establish its mechanism, while at the same time identifying test conditions that can be used for an industry wide acceptance test for whisker formation, since this test has never been established.At the beginning of the study, we found different causes for whisker growth depending on the leadframe material; in case of Cu leadframes irregular growth of Cu 6 Sn 5 was identified as the main cause for whisker growth during isothermal storage, while on FeNi42 leadframe the mismatch of coefficient of thermal expansion caused whisker growth in temperature cycle tests.Within the E4 postbake (1 h, 150 o C) was introduced as a countermeasure for whisker growth for Cu based leadframes.Since last year many different test proposals for whisker testing have seen the light of day, resulting in a variety of test conditions. The E4 have submitted their solution, this postbake at 150 o C for 1 hour within 24 hours after plating, to most of the proposed tests. For all test conditions the results were positive (no whisker growth) up to 3000 hours. In this paper the test results are presented and discussed.Recently, whisker growth was discovered in some cases after prolonged exposure to 60 o C/93%RH on bare components, regardless of preconditioning. These whiskers were observed in combination with severe corrosion. However, on board assembled units no whiskers or corrosion were observed. In order to understand the influence of humidity and corrosion on whisker growth the E4 started an extensive Design of Experiments (DoE). Parameters in this DoE include: plating, solderpaste, flux, substrate material, preconditioning and condensation.In this paper we will present the outcome of this DoE and furthermore give an update on the previously reported and ongoing experiments to discover the main factors influencing whisker growth.

Tin Whisker Test Method Development

“…Su and coworkers observed that decreasing the humidity from 93%RH to 85%RH in a 2000-h exposure at 60 C reduced the number of leads with whiskers by a factor of 10, decreased the whisker lengths, and prevented localized corrosion [17]. Other researchers observed similar results for 85%RH/55C test conditions [18].…”

“…Corrosion may cause a new whisker mechanism or may alter factors involved in whisker growth, such as stress [18] or diffusion. However, corrosion is not the only explanation for whisker growth, since we observe whiskers in ambient testing and thermal cycle testing, which does not create localized corrosion.…”

Synthesis, characterization and in situ Raman detection of Sn 3 O 2 (OH) 2−x Cl x phases as intermediates in tin corrosion

“…Especially halides and flux residues after soldering processes are presumed to function as initiators for chemical reactions on the metal surface [6]. Corrosion products located between columnar deposited grains of a tin surface are presumed to induce significant stress levels, which initiate whisker growth [5,6]. To prove this model, it is of utmost importance to analyze the corrosion process itself and to know which tin(II) and or tin(IV) chemical species are located on the metal surface to induce stress levels.…”

Growth of tin whiskers for lead-free plated leadframe packages in high humid environments and during thermal cycling

“…When considering applications of high reliability such as automobiles, we need to understand the exact mechanism of the whisker growth in high humid environments and during temperature change, and then provide feedback to package design as well as plating and assembly processes to minimize whiskering. However, until now, there have been few studies on whiskering in these two conditions, especially for alloy42 LF packages [3][4][5][6][7].…”

“…In the ambient storage condition, as the result of the formation of Cu 6 Sn 5 intermetallic growth in tin grain boundaries, compressive stress is generated inside the tin film, thereby atoms move from the highly compressive stressed film/substrate region to the near-zero stressed whisker grains through grain boundaries [2]. In the elevated temperature and high humid environments oxidation and corrosion of tin-plating layer are the root-cause of the whiskering [3,4]. Osenbach et al identified the corrosion products as dense mixture of crystalline SnO 2 and Sn by transmission electron microscopy analysis and concluded that injection of oxygen into the tin film creates localized compressive stress [4].…”