Recently, an EMPF customer called to discuss failures that they had observed with some through-hole capacitor parts. The components were experiencing failures following vibration and accelerated stress testing.

Upon receipt of the samples, the EMPF performed a series of tests to investigate the root cause of the failures. The samples were run through three levels of visual inspection and one test to verify the elements comprising the solder joints. The first inspection was to investigate the capacitor leads. Using an Olympus microscope, no anomalies were found that could indicate bad parts from the vendor or improper handling prior to assembly. We did note, however, that the amount of solder fill in the visible barrels of the plated through-holes approached the IPC-A-610 minimum specification of 75%.

Next, the assemblies were placed in the Dage X-ray. This non-destructive test allowed us to see inside the solder joints before doing irreparable damage to our samples. From the x-ray images, the EMPF was unable to get a complete picture of the barrel fill at the failed junctions. It was readily apparent that the solder was cracked at the source side of the failed joints. Now we needed to find out why.

The assemblies were then cross sectioned and placed in our Scanning Electron Microscope to examine the failures under high power magnification (over 100x). This tool allowed us to look for micro-cracks in the solder and the proper formation of intermetallic layers at the solder to copper junction. After micro sectioning, it was evident that insufficient solder fill was the failure mechanism for these capacitors. Both sides in the plated through-hole appeared to have very little solder (Figure 2-1), allowing a large gap between the lead and the barrel. In addition, the opposite lead of capacitor C12 (that appeared to be intact after testing) was actually damaged, with a fracture crack as shown in Figure 2-2. Multiple voids were observed from the cross-section in both optical and SEM images which showed micro-cracks bridging together.

According to IPC-A-610D standard, there must be a minimum of 75% solder fill for Class 2 and 3 products. The opposite lead of capacitor C12, which showed the barrel with moderate solder fill, was used as the control even though fracture cracks were observed in the solder fill. This control capacitor lead barely passed the IPC-A-610D standard. For the fillet and wetting on the solder source side of the lead and barrel, there was 360º wetting present which passed the minimum 270º wetting requirement for Class 1, 2 and the minimum 330º wetting requirement for Class 3. Also, the percentage of land covered by wetted solder on the solder source side was 100% which passes the minimum 75% land area for Class 1, 2, and 3 products.

The EMPF analyzed each micro sectioned sample using Energy Dispersive Spectroscopy (EDS) to determine the composition of the intermetallic bond in the solder joints. EDS showed the existence of a Cu-Sn-Ni intermetallic layer between the interface of the bulk solder and the plated through-hole (Figure 2-3). This underdeveloped intermetallic layer could be another sign that the solder dwell time was insufficient to allow the solder to wick into the plated through-hole for maximum fill.

Based on these findings, recommendations were made to the customer to aid in controlling their process.

• The plated through-hole diameter/lead diameter ratio for the electrolytic capacitors needs to be re-evaluated. Low clearance for the capacitor lead in the plated through-hole may inhibit the solder flow to fill the hole. The EMPF referenced the IPC-2222 design guidelines for rigid printed board with standards for hole/interconnections (Section 9). The manufacturer specification should also be consulted for the appropriate plated through-hole diameter/lead diameter ratio.
  
  
• The solder process should be re-evaluated. Barely passing the IPC requirement for 75% minimum solder fill in plated through-hole indicates that there are several areas to investigate. Increasing the pre-heat temperature, lengthening the solder dwell time, and increasing the amount of flux application can improve the solderability of the capacitor leads and achieve the IPC target hole fill of 100%.

For more information on through-hole manufacturing and reliability, or other classes available from the EMPF, including IPC certifications, please contact the registrar at 610.362.1295 or via email at registrar@empf.org. A complete listing of course descriptions are on the web at www.aciusa.org/courses. For a demonstration of the Dage X-Ray or Scanning Electron Microscope, please contact the EAB coordinator, Ken Friedman at 610.362.1200 extension 279, or kfriedman@aciusa.org.