Solderability Testing Aids Selective Soldering Process Development


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ACE Production Technologies' Selective Soldering Applications Lab recently processed a group of samples for a customer; in the parts mix was a component that had oxidized leads. During process development, the operator found that no matter which flux was selected, or which profile was used, the solder would not wet to the part. Process development for the application was stalled by the solder dewetting. Solderability tests were performed to analyze the wetting curve and determine the cause of wetting failure. 

Figure 1. X-ray after first attempt.We initially processed the customer's sample assemblies using the KISS-103 selective soldering system. A 4.5-mm soldering nozzle, the customer-provided 2009M no-clean flux, and Alpha SAC 305 lead-free solder were used. The 4.5-mm nozzle was chosen for its inherent precision, and it also provided the width needed to solder the pins in the least number of passes. This nozzle also provided an ample amount of thermal energy to heat the parts, allowing solder to flow for a minimal dwell time. Flux application was done with a drop-jet fluxer, which applies a precise amount of flux to a specific location, reducing overspray and providing a cleaner finish.

After processing the first board, X-ray images were taken of the barrel fill (Figure 1) that looked good for a first attempt, and were assumed easy enough to correct where 

Figure 2. Pins showing non-wetting.needed. There were also indications that some adjustments might be needed to achieve a proper bottom-side solder shape. Adjustments were made to the flux application, as the shapes indicated that there was a lack of flux getting to the pins, causing some bridges and "flagging." The lab re-ran the same board with these changes made, and the bottom-side result was much improved.

The second board was soldered using the same program. This process run delivered a nearly identical result to that of the first, showing good barrel fill, but with bottom-side shapes that were undesirable. Figure 2 shows examples of pins resisting the solder wave almost completely. Only when the wave was able to contact the pad itself did any soldering take place. Based on the X-ray of the switches (Figure 3), which shows voided areas and, significantly, "spotted" areas of the pin where the solder did not adhere, the lab technicians concluded that there was a

Figure 3. X-ray void and poor pin solder acceptance. contamination problem. They questioned whether or not there was any intermetallic bond created between the solder and the pin.

After a couple of additional attempts to correct the remaining bottom-side pins, a wetting balance (solderability) test was performed with a a MUST III System, a wetting balance testing system capable of testing to all relevant solder standards including ICE, MIL-STD, and IPO/EYE/JADE. The system measures the solderability of a printed circuit board  and/or component's metallic terminations by documenting the wetting curve of the unit under test (UUT). It is appropriate for testing multi-leaded components, SMT and BGA devices, and other components.

Nine of the pins were tested, and the results recorded (Figure 4). No flux was used for the first pin, providing a worst case scenario. The remaining eight pins were processed using the same flux that was provided for processing the sample boards. As may be seen, most of the pins did not recover beyond the "zero point" or "buoyancy" level, which is the point where the pin stops resisting the solder and begins to be drawn into it. Figure 5 shows the graph of another set of tests conducted with the exact same parameters and pins from another sample part in the lab's inventory (not one of this customer's samples). There is a significant drop in initial solder rejection, as well as attaining "maximum" acceptance within 0.5 sec. The lab re-tested the four weakest results (Figure 6) from the first graph, which proved that the newly tinned pin accepted solder much better, closer to normal expected results, proving that the plating condition was responsible for the bottom-side shape problems.

Figure 4. Initial part testing of nine pins.

For the final run, all of the leads were trimmed to approximately 1 mm to see if the solder might overwhelm the oxidized plating and provide an acceptable result. This proved insufficient to offset the problem. We also concluded that at no point was the barrel plating material an issue, since even from the first run we had no trouble getting full visible topside.

Figure 5. Testing of similar pins with identical parameters.

ConclusionSoldering defects due to non-wetting have a variety of causes, and those causes must be identified and their root problems solved to realize the benefits of automated production and achieve high yields with steady repeatability. Solderability issues regardless of the source (e.g., plating) can be identified, tracked, and solved with good solderability testing and the selective or other soldering process then optimized.

Figure 6. Retest of four weakest pins. Greg Goodell, process development technician, A.C.E. Production Technologies, 3010 N. First Street, Spokane Valley, WA 99216, may be contacted at (509) 924-4898; ggoodell@ace-protech.com.

See Related Articles:The Basics of SolderingChris Nash, Indium Corporation, presents a tutorial-style look at solder and soldering processes.

ACE Opens Selective Soldering Development LabACE's lab hosts all of ACE's selective soldering tools, including traveling mini-wave systems, advanced solder fountains, spray and drop-jet fluxing systems, the LTS 200 lead tinning system, and a nitrogen generator. The Lab is also equipped with X-ray equipment and solderability (wetting balance) and cleanliness (ionic) test systems.

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