Circuits Assembly Online Magazine - A Novel Process for Automated Lead Tinning

Components with Sn99 or other whisker-prone finishes can now be rapidly de-taped, tinned, cleaned and re-taped.

Component finishes made up primarily of Sn99, with the remaining alloys being silver, nickel, copper, germanium or some combination thereof, have been known to result in tin whisker growth, increased tombstoning of smaller chip capacitors and resistors, reduced shelf life of components (a reduction in solderability over time) due to Sn99’s higher oxidation rates versus leaded finishes, and other issues.

While certain high-reliability products are exempt from RoHS and similar directives, all are ultimately affected as a result of component vendors’ desire to provide a common Sn99 finish. It is not economical for component suppliers to provide the same components in both Sn63 and Sn99 finishes. Gold as a final finish is seeing increased use as a protective coating to prevent oxidation, but must be removed in high-reliability soldering applications. As more components come only with gold finish, the need for rapidly de-taping, tinning to remove the gold using a dynamic wave, and then re-taping becomes more pressing.

Tin whiskers increasingly are problematic. Some studies detail the probability of tin whisker bridging.1 While high-reliability military, aerospace, avionics, traffic control, medical and industrial equipment are exempt from the RoHS Directive (and for good reason), all need to incorporate some type of whisker mitigation method to disposition Sn99-finished parts. Industry standards for high-reliability electronics specifically require whisker mitigation methods (tinning with at least 3% lead) be used for any components with a Pb-free finish, and the tinning must cover 100% of the component lead, not just the part to be soldered.2 Any gold on the component leads, no matter the thickness, shall be removed to prevent gold embrittlement. To achieve these objectives, many rely on some sort of tinning and component repackaging service, either performed in-house or by an outside provider. This traditionally has been a manual process due to the wide variety of component types and requirements, but increasing volumes and process variability are quickly rendering manual tinning costly and impractical. Here are some of the reasons:

To prevent tin whisker growth, the entire portion of the component lead or termination must be coated, either with a SnPb alloy, or some other stress-mitigating alloy. The tinning solder must contain at least a 3% Pb alloy to be effective as a whisker mitigation method.
At the same time, the molten solder should never make direct contact with the component body for more than two sec. to prevent damage to internal connections or lead-frames, which may lead to reduced reliability.3
To apply solder coating over the entire component lead or termination right up to the component body without damaging the component, the immersion dwell time and depth must be precisely controlled so as to permit solder to wick up the last 0.003˝-0.005˝ of the termination, thereby preventing thermal shock damage to the component and leaving no portion of the Sn99 finish exposed. This requires a computer-controlled automated system. In addition, the tinning flux must be of the type that will remain on the component lead all through the tinning cycle to ensure good wetting.
The solder level must be precisely controlled, such that an automated system can be fully utilized. Attempting to manually fill solder to a precise level within a pot, or to manually dip a component with the required precision and hold it for a dwell time measured in milliseconds, is humanly impossible.

As mentioned, oxidation issues with older Sn99-finshed components are just beginning to materialize. Because pure tin oxidizes much more rapidly than a lead-containing alloy, Sn99 parts have a shorter shelf life before wetting defects begin to add to the DPMO of any soldering process. In addition to the simple non-wetting or poor wetting defects, tombstoning can become a major issue when using oxidized Sn99 chip capacitors and resistors because either end may not wet as readily as the other, and this imbalanced wetting action is what causes the component to be pulled toward one pad or the other.

For those companies required to meet the Pb-free portion of the RoHS restrictions, tinning with SnPb37 solder to prevent whisker growth is not an option.

However, alloys such as Kester’s K100DL or Nihon Superior’s SN100C4 are doped with trace amounts of other alloys that may help inhibit tin whisker growth. These alloys can be used to tin components within the robotic tinning cell and will renew the component solderability of oxidized Sn99 components as well.

For these reasons, General Dynamic’s Advanced Information Systems group approached V-Tek to build a robotic tinning cell using a precise component handling system (Figure 1).

The robotic tinning platform performs the following steps:

An Epson robot picks the component from either a standard eight to 45 mm tape fed into the de-taper, which is any feeder for most pick-and-place machines, or any JEDEC matrix tray. After processing the parts, a Hover-Davis re-taper is used to re-tape the parts. This TM-50 unit is integrated into the robotic platform and controlled by the robot software. Matrix tray locations are taught into the robot software (Figures 2 and 3).
The V-Tek robot is set up to use Mydata nozzles or those from other common pick-and-place machines.
The robot then performs a process control inspection of the previously formed component leads using a Coherix 3-D camera system to inspect for bent or abnormal component leads, lead coplanarity, toe-to-toe span and heel-to-heel span to within 0.001˝. Rejects are placed in a reject tray for disposition or rework.
After 3-D inspection, the robot applies a halide-free water-soluble flux specially formulated for automated tinning. This flux prevents bridging on fine-pitch leaded components.
Next, the robot dips the component into a nitrogen-blanketed dynamic (flowing) solder pot (Figure 4). Immersion can be done within 0.001˝ z-axis accuracy for programmed increments of time down to 1 ms, and with the entire spectrum of component package styles such as SOICs, QFPs, SOT-X, connectors, and leadless chip capacitors and resistors down to 0402 size. The part can be dipped at virtually any angle, depth and dwell.
After the tinning step is completed, the robot precisely holds the component into a hot deionized water wash system that is an integral part of the platform (Figure 5), and then presents the component to an air knife system for drying.
The robot then brings the tinned and cleaned component over a second Coherix 2-D inspection system (Figure 6) to verify there are no solder bridges, icicles or excess solder, insufficient solder coating, or other solder defects (Figure 7).
Next, the robot places the finished component back into a new tape or matrix tray or reject tray for disposition/rework (Figure 8). The tray position is programmable and can be taught to the machine.
As previously described, de-taping of components is performed using pick/place feeders clamped onto the robotic platform, and the robotic software controls sequencing. Feeders from most major pick-and-place machines can be used.

The system can handle small quantities of components quickly and efficiently to meet the needs of flexibility when running small lots. In addition, the robotic tinning cell can be used for many other applications. One example is to use the robot to dip BGAs into the solder pot to a precise depth to remove the old solder balls as part of a BGA rework/reballing process.

Acknowledgments

The authors would like to thank Mark Jensen and George Andreadakis of V-Tek, and Dan Volenec and Mike Soltys of General Dynamics AIS, for their long and hard work in the development, debugging, programming and qualification of the Robotic Tinning machine.

References

1.   S. McCormack and S. Meschter, “Probabilistic Assessment of Component Lead-To-Lead Tin Whisker Bridging,” International Conference on Soldering and Reliability, May 2009.
2.   J-STD-001DS, “Space Applications Electronic Hardware Addendum to J-STD-001D Requirements for Soldered Electrical and Electronic Assemblies,” September 2009.
3.   Shirsho Sengupta and Michael G. Pecht, “Effects of Solder-Dipping as a Termination Re-finishing Technique,” doctoral thesis, August 2006.
4.   Keith Sweatman, personal communications.

Richard Stadem is a principal process engineer at General Dynamics Advanced Information Systems (gd.com); richard.stadem@gd-ais.com. Cornel Cristea is director of engineering at V-TEK Inc. (vtekusa.com).