Critical Considerations for Tin Whisker Mitigation
Andy Price, Bob LePage, David Cormier, Jim Rennick
Circuit Technology Center, Inc.
Haverhill, MA USA
This paper provides an in-depth analysis of the issue of tin whiskers and offers valuable insights into effective mitigation strategies. The paper highlights the potential risks of tin whiskers, including their role in causing electrical shorts and system failures in electronic components. The paper emphasizes the importance of proactive mitigation measures by exploring factors influencing the growth of tin whiskers, such as material composition, stress levels, and environmental conditions.
The paper examines techniques like conformal coating, mechanical reinforcement, and alloying to minimize tin whisker formation. This paper is a valuable resource for engineers and researchers in the electronics industry, offering critical considerations and practical guidelines for mitigating the detrimental effects of tin whiskers.
The phenomenon of tin whiskers in printed circuit board assembly is a failure mechanism associated with electronic devices that use various solder alloys containing low melting point elements such as tin, cadmium, or indium. However, this phenomenon most commonly occurs with tin. Historically, tin whiskers were avoided by adding lead (Pb) to the solder alloy used for component leads or pads and circuit boards with HASL (hot air solder leveled) finish. However, since lead has been identified as a hazardous substance and has been banned for use in many electronic devices, tin whiskers are a real threat.
Tin whiskers are a reliability concern since tin whiskers are conductive and can carry a high current. Without lead, the material used for the past 50 years to limit whisker growth; this failure mechanism issue can affect most current electronic applications.
Whiskers will grow from the surfaces of copper electronic device leads, pads, or copper substrates, finished with low melting point solder alloys containing tin, cadmium, indium, zinc, or antimony. Research has shown that whiskers will grow from tin-lead surfaces under certain conditions, but the length of these whiskers is typically shorter due to the presence of lead.
Unlike tin pest, a failure mode that occurs solely in extremely low-temperature environments such as high-altitude aerospace applications, tin whiskers can occur at ambient temperatures. Tin whiskers have grown to a length of 0.025” (0.635mm) on 100% bright tin-plated connector leads stored for approximately four months at ambient conditions. In other environments, tin whiskers have grown to 10mm in length.
An often-asked question is if a conformal coating will prevent the growth of tin whiskers. No known conformal coating will stop a tin whisker from emanating from a tin-plated surface. However, a properly applied conformal coating will prevent electrical shorts associated with tin whiskers. It has been determined that Parylene C and silicone coatings are the most effective at suppressing tin whisker growth, while acrylics are typically the least effective. The difference is the hardness of the coating, as harder coatings tend to perform better at stopping tin whisker propagation due to the greater force needed to penetrate these harder coatings by a tin whisker.
There are certain conditions that promote tin whisker growth, including, but not limited to, thin tin plating, residual stresses during the tin plating process, or insufficient intermetallic compound formation during plating. All tin-plated copper alloys experience the formation of copper-tin intermetallic compounds, either Cu6Sn5 or Cu3Sn, at the interface of the tin and the base metal. Thin tin plating is more susceptible to whisker growth since thin plating develops greater compressive stresses than thick tin plating. Mechanical defects in the plating surface, such as nicks and scratches, can accelerate tin whisker formations.
Robotic system using tweezers to grip discreet components for tinning.
Based on iNEMI research, it is their recommendation to use printed circuit boards with a board finish of either nickel palladium gold, nickel palladium, electroless nickel immersion gold (ENIG), or nickel gold to reduce the risk of tin whiskers originating from the copper surfaces of the circuit board itself. Alternatively, a matte tin finish on printed circuit boards can be used, providing the plating has a minimum thickness of 6µm (microns).
A more reliable method to mitigate tin whiskers is robotic hot solder dip processing of components before circuit board assembly. This process removes 100% of the pure tin plating from the leads or terminations and replaces it with tin-lead, preventing tin whisker formation. Robotic hot solder dip processing can be performed on all through-hole and surface mount components, including axial, radial, SIPs, DIPs, SOICs, SOTs, QFPs, plus through-hole and SMT connectors, as well as discreet electronic devices.
Robotic PLCC component tinning.
The use of fully programmable robotic hot solder dip machines is highly recommended instead of manual solder dipping since these robotic systems precisely control the solder dip depth, dwell times, preheat, and solder temperatures in full compliance with GEIA-STD-0006 standards. The GEIA-STD-0006 standard, Requirements for Using Solder Dip to Replace the Finish on Electronic Piece Parts, states that robotic solder dipping apparatus shall have:
- Dynamic solder wave or another method to remove oxidation before solder dipping
- Controlled dwell time in preheat and solder pot within ± 0.1 sec
- Controlled depth of immersion to within ± 0.1mm
- Controlled exit speed out of solder pot to within ± 0.3 cm/sec
- Piece parts shall be pre-heated to no less than 71°C prior to solder dipping
- Total immersion time shall be less than 5 seconds per each component side
Robotic hot solder dip tinning services are available to change the metallic finish of through-hole or SMT component leads or pads. These services commonly change leads or pads from a lead-free solder finish to a tin-lead solder finish for tin whisker elimination. When using these robotic hot solder dip tinning services, it is recommended to also use a batch wash system for post-process cleaning as well as the following procedures:
Robotic QFP component lead tinning.
The following testing services are beneficial and recommended to ensure process integrity:
- Component moisture sensitivity level (MSL) dry bake per J-STD-033
- Component moisture sensitivity level (MSL) packaging per J-STD-020
- Tape and reel packaging per EIA-481
For ultra-high reliability, mission-critical applications such as military, security, defense, and/or aerospace, additional component testing services can be required using the following test protocols:
- Ionic cleanliness (ROSE) testing per IPC-TM-650-2.3.25
- X-ray fluorescence (XRF) for alloy composition and finish thickness per JESD 213
- Solderability testing per J-STD-002
- Visual inspection
- C-SAM (scanning acoustic microscopy) testing per J-STD-035
- Destructive physical analysis (DPA) per MIL-STD-1580
- Hermeticity testing (fine and gross leak) per MIL-STD-883
- Temperature, humidity, and bias testing
- Parametric testing
The paper highlights the significance of proactive measures in mitigating this phenomenon by addressing the challenges associated with tin whisker formation, such as the potential risks to electronic devices and systems. The authors emphasize the importance of understanding the underlying mechanisms of tin whisker growth and the role of environmental factors in implementing successful mitigation techniques.
With a focus on industry standards and emerging technologies, this paper serves as a valuable resource for professionals and researchers seeking to address the issue of tin whiskers and safeguard the reliability of electronic systems.