SMT Electrolytic Capacitor Solder Joint Criteria and Integrity Investigation

SMT electrolytic capacitor is a widely used component technology in the design of IPC Class 3 avionics. The main structure of the SMT electrolytic capacitor results in the solder joints being only partially visible for optical inspection purposes. Figure 1 shows a blurred optical view of the capacitor wires due to the body configuration/wire configuration. The capacitor leads are led out from the bottom of the assembly, and the "L" shape is bent into a horizontal plane to provide surface area for surface mounting solder joints.  

A typical SMT electrolytic capacitor. (Photo provided by Panasonic)

Solder joint process standard: according to IPC-JSTD-001 specification, gull wing shape (top) and lug (bottom).

Controversy/discussion about whether this lead configuration falls under the gull wing or the acceptance standard for lug lead optical solder joints (Figure 2) has begun. Two key issues arose during internal discussions on this issue:

If enough solder paste can be printed to ensure that acceptable heel and side fillets are created, the discussion of the entire solder joint process will be controversial. This method is not a viable option because the large amount of solder can cause solder joint bridging and capacitor placement defects.

The problems associated with SMT electrolytic capacitors are not limited to Rockwell Collins. The IPC-JSTD-001 National Standards Committee established a task group to study and create the solder joint acceptance standards for SMT electrolytic capacitors. The JSTD-001 task force is composed of avionics/commercial electronics assembly manufacturers and component suppliers. The working group created a set of proposed welding process standards for SMT electrolytic capacitors (Figure 3). Rockwell Collins (Rollwell Collins) generally agrees with the proposed process standard, except for the minimum heel fillet height (G (the thickness of the solder under the component lead) + T (the thickness of the component lead)), which is internally discussed The highlight is the solder joint properties and needs further investigation. Note 3 in the following table is only the solder thickness under the component leads.

SMT electrolytic capacitor is a standard component and has been widely used in many avionics products for several years. Therefore, for any concerns about whether the capacitors will meet Rockwell Collins soldering process standards after years of production, it is more the issue raised by new product auditors rather than changing the solder reflow process. Two years of SMT electrolytic capacitor defect data were plotted to determine the impact of any type of process/product/personnel change (Figure 4). Since the reflow soldering process or component configuration changes are not recorded, the defect changes shown in Figure 4 are subjective, inconsistent solder joint process standard visual inspection results (ie, the solder height is expressed as a percentage of the lead thickness (T) – The height of the fillet on the solder side is 1/4T or 1/2T or 1T?). Reviewing the product field failure data of SMT electrolytic capacitors, it was found that there were unreported defects out of 10,000 opportunities in 8 years.

 IPC JSTD 001 SMT Electrolytic Capacitor Proposed Process Standard Draft (Credit: Jim Dagget, Raytheon and IPC JSTD-001 SMT Electrolytic Capacitor Working Group).

Work together with the IPC-JSTD-001 working group to establish solder joint acceptance standards for SMT electrolytic capacitors. Rockwell Collins (Rockwell Collins) initiated an investigation to determine the key properties of solder joints through thermal cycle adjustment and shear testing. The test results will be used to create a set of recommended solder joint acceptance criteria for inclusion in the "Rockwell Collins Process Standard" and will be reviewed by the IPC-JSTD-001 working group.

Created a test plan for two solder joint properties: (1) the mechanical strength of the solder joint; (2) the thermal cycle fatigue strength of the solder joint. One of the main reasons for having solder joint geometries and wetting standards is to ensure the creation of consistent and repeatable solder joints that can withstand the standard mechanical stresses of the product's use environment. Although the main purpose of component soldering is to provide electrical connections, some mechanical properties are still desired. Due to its geometry, traditionally, electrolytic capacitors require additional mechanical reinforcement and use adhesives for vibration or shock purposes. The use of adhesive bonding methods can reduce solder joint geometry and wetting requirements. Shear tests are included in the test plan to prove that board pad connection failure occurs before the solder joint fails. The test includes solder joint thermal fatigue testing because it represents the main failure mode of solder joints in avionics applications. The electrolytic capacitor will be adjusted for thermal cycling from -55°C to 125°C, and then metallographic cross-sectional analysis will be performed to determine the degree of solder joint cracking caused by the overall coefficient of thermal expansion (CTE) mismatch caused by thermal cycling.

 The process inspection of SMT electrolytic capacitors for solder defects lasts for two years.

The test tool used in the study is a typical printed circuit assembly, which contains 10 SMT electrolytic capacitors, making it ideal for testing (Figure 5).

The SMT aluminum electrolytic capacitors analyzed in the survey represent the SMT electrolytic capacitors used in the industry. Rockwell Collins purchases these components from various industry component suppliers (Figure 6, Figure 7). The pad configurations between the various suppliers are similar, so the resulting solder joints are very similar. Since these components are easily damaged by vibration in the environment where the product is used, an enhanced gasket configuration is sometimes used in auxiliary silicone adhesive bonding (Figure 7, anti-vibration). 

 A component showing the location of an SMT electrolytic capacitor.

A typical SMT electrolytic capacitor. (Photo courtesy of Conrad Dubilier)

The test vehicle was assembled at the Rockwell Collins Colleville production plant in Iowa. The solder paste is Henkel MP-218 tin-lead eutectic solder deposited with a 0.005 inch thick template. Use Universal Advantis automatic placement machine to place components. Reflow the components in a Heller 1912EXL reflow oven. Clean the components in the Electrovert Aquastorm 200 in-line cleaner. Due to the packaging structure, most of the solder joints are covered, and only the outer ends of the component leads can be seen through optical inspection (Figure 8). The parts of the solder joints that are visible to the naked eye are considered unqualified because they do not meet the minimum fillet height of 1T. The typical placement/spacing of SMT electrolytic capacitors makes solder joint repair challenging and creates potential opportunities for component or laminate damage.

The test vehicle is placed in a hot chamber set to a temperature range of -55°C to 125°C. The ramp rate is set to 5°C–10°C per minute and held for 10 minutes at each temperature limit.

The temperature profile is shown.

 A typical SMT electrolytic capacitor. (Photo provided by Panasonic).

After completing a total of 532 thermal cycles, the components were taken out of the thermal cycle chamber for metallographic cross-sectional analysis.

Before the thermal cycle test, a set of electrolytic capacitors was carefully taken out of the assembled test car for shear test. This data set records typical welding conditions rather than thermal cycling conditions to provide a measure of solder joint wetting/geometric strength. An Instron test system with a shear rate of 0.15 inches/minute was used for testing.

Explains the test component/cut test setup. The average peak load measured by the shear test was 40.3 pounds, and the standard deviation was 5.4 pounds (

). There is no industry-recognized minimum shear value, but 30 pounds is considered very suitable for the integrity of the welded joint.

For comparison, a typical component pad peel strength is 6-12 pounds.

 Optical view of solder joints after automatic reflow soldering.

After the shear test, the failed solder joints were optically inspected. It was found that the solder joints had few voids, wetting/soldered joint geometry was good, and the solder joint/component lead interface and solder joint/component pad interface had complete metallurgical adhesion (Ie cohesive destruction).

 It illustrates the typical failure solder joint interface caused by the shear test.

After completing the thermal cycle adjustment for metallographic analysis, many SMT electrolytic capacitors were removed from the test vehicle. Some very small cracks were observed in the fillet area behind the solder joints, but no abnormalities in the microstructure or cracks of the solder joints were observed, indicating that the integrity of the solder joints was reduced due to thermal cycle adjustment (

).

It should be noted that the use of standard automatic reflow soldering process cannot achieve "1T" solder joints followed by rounded corners or side rounded corners. It is found that the height of the fillet behind the solder joint is within 1/2T.

Figures 14 to 16 illustrate the height of the solder fillets using a standard automatic reflow soldering process.

A toe cross-sectional view of an SMT electrolytic capacitor, showing ~63%T side fillet height.

A cross-sectional view of the toe of an SMT electrolytic capacitor, showing a ~54% T side fillet height.

A cross-sectional view of the toe of an SMT electrolytic capacitor, showing ~48%T side fillet height.

The shear and thermal cycle test results clearly show that the solder joints of SMT electrolytic capacitors have sufficient solder joint mechanical and thermal fatigue properties. The average value of 40 pounds proves that the solder joints are strong, and the metallographic evaluation shows that all solder joint interface areas have excellent wettability. After 500 thermal cycles, the solder joints did not crack/fail, indicating that these components meet the minimum requirement of 500 trouble-free thermal cycles required by the traditional Avionics Management Program (ECMP). Metallographic cross-sectional analysis shows that the fillet and side fillet of the weld joint produced by the automatic welding process are about 1/2T. The practical limitations of visual inspection have caused inspectors to mistake the fillet height as 1/4T. For the tooth-shaped solder joint fillet geometry on the SMT filter, a similar visual inspection problem was found [1]. Figures 17 and 18 illustrate how visual inspection is too conservative when checking the fillet height of the solder joint. According to visual inspection, the measured fillet height of the solder joint is 75% instead of 50%. The inspection and "perceived" solder joint fillet height of SMT electrolytic capacitors is similar to the "reached" solder joint fillet height. Visual inspection of solder joint height can produce "false negative" responses, which can lead to unnecessary component rework. Since the automatic reflow soldering process will always produce acceptable solder joint integrity, setting the heel and side chamfer height requirements of the solder joint to 1/4T will not be a problem. Similarly, due to the nature of the process, manual soldering of SMT electrolytic capacitors will always produce more than 1/2T of the heel and side fillet height of the solder joint.

 The audited optical inspection solder joint height is less than 50% of the solder fillet height [1].

 The measured metallographic cross-section solder joint height is equal to 75% of the solder leg height [1].

Tests show that the reliability requirements can be met without the need for 1T solder joints followed by fillets and side chamfers of solder joints. For the standard welding process used by Rockwell Collins products, the minimum 1/4T requirement is sufficient to meet the height requirements of the heel and side fillets of the solder joint.

The research on SMT electrolytic capacitors is to answer two main questions:

Shear testing, thermal cycle testing, and metallographic cross-section results show that the automated reflow process can create solder joints with acceptable solder joint integrity. Defects in visual optical inspection lead to "false negative" assessments of solder joint quality.

It is recommended to revise the industry process standards of SMT electrolytic capacitors to the following standards:

The height of the minimum solder joint followed by the fillet and the side solder joint of the SMT electrolytic capacitor should be 1/4T.

The author would like to thank the Coralville Common Process team for testing vehicle assembly, Richie Korneisel, Will Quandt and Ben Theile for metallographic cross-section processing, and Ross Wilcoxon for severe criticism of the manuscript.

1. D. Hillman et al., "Investigation of welding joint process standards for parts with toothed welding joint configuration", Rockwell Collins working paper, WP12-2001, 2012.

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