Critical Considerations for Tin Whisker Mitigation
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 been found to grow 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.
Robotic system using tweezers to grip discreet components for tinning.
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.
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).
Robotic PLCC component tinning.
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.
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 71⁰C prior to solder dipping
Total immersion time shall be less than 5 seconds per each component side
Robotic QFP component lead tinning.
Robotic hot solder dip tinning services are available that can 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:
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
The following testing services are beneficial and recommended to ensure process integrity:
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
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:
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
Several members of the Circuit Technology Center team contributed to this feature story.