Developing a New Aluminum Electrolytic Capacitor Holder for
Ruggedized Automotive Applications
By Tom Anderson, AVX Product Manager

Harsh environments such as those surrounding automotive and industrial applications can be very demanding on larger electronic components, which are exposed to severe shock and vibration requirements. In order to protect these components, extra attention, and additional costs need to be devoted to the processing and mechanical attachment within the device to the printed circuit board (PCB).

A new product is needed to meet these demands, yet the process by which new products are brought to market is increasingly complex. How do we meet the customers’ needs while navigating this minefield? In one specific case, the challenge was to develop a reliable and cost-effective solution to terminate an 18mm diameter aluminum electrolytic capacitor to the PCB. The goals in this application included:               

1. Develop a mechanical holder that would securely hold and electrically connect the capacitor to the PCB, while providing mechanical attachment of the completed module to the PCB to meet specified drop, impact, and vibration testing.

2. Develop a contact system for both the capacitor and PCB terminations, which will meet specified electrical and environmental testing requirements.

3. Provide a simplified assembly process for the completed module, which can be applied after the reflow process. As a note, the capacitor cannot be exposed to the elevated temperatures of the RoHS soldering process.

The solution was to identify the contact system, as this would become the heart of the entire module. To do this, AVX Interconnect combined two contact technologies. This marriage of a press-fit compliant pin for the PCB and insulation displacement for the capacitor leads would be able to meet all of the goals outlined above. Both of these contact systems have been qualified and used in multiple automotive connectors over recent years. Here is a quick overview of each technology.

Press-Fit
The eye-of-the-needle compliant pin has been in production since the mid-1970s. The updated hourglass-shaped compliant section was specifically developed for the automotive industry to better distribute the stress over the entire length of the press-fit section. This resulted in lower PCB stress, an enlarged contact area, and increased tolerance accommodation in the plated through hole. During optimization of the hourglass cross section, special care was taken to produce minimum insertion force with maximum retention force across the entire spectrum of plated-through-hole diameters. Specific advantages include:

1. 100 percent elastic press-fit cross section

2. Automotive shock and vibration performance

3. Selective plating

4. Low transition voltage

Insulation Displacement
Insulation displacement connectors (IDC) have been around since the early 1980s. As the name implies, a phosphor bronze contact would pierce the insulation of a wire, wedging the core conductor(s) into the “V” slot of the contact to create a gastight connection. The only difference with the capacitor holder is that there is no insulation on the leads of the capacitor. The primary purpose of the insulation in a traditional IDC wire termination is to provide a strain relief to protect the contact integrity when there is pulling or bending forces applied to the wire. When used within the holder, the capacitor is held firmly within a retention cradle that immobilizes the capacitor, and thus, protects the integrity of the gastight termination. The use of IDC-style connectors today spans a broad range of sizes and requirements for automotive and harsh environments, including:

1. 14AWG to 28AWG

2. 1 to 15 amps

3. Solid wire

4. Stranded wires

The next step was to develop the robust plastic housing that would retain the contacts and capacitor in a single module that could be cost effectively pressed into the PCB. A 30 percent glass-fiber reinforced polybutylene terephthalate (PBT) thermoplastic polymer was selected because of its mechanical strength and heat resistance characteristics. Integrated into the molded housing are very special features that are critical to meeting the final mechanical performance criteria of 1.2 meter drop test, 50g impact test, and temperature-based vibration testing. These include: 

1. Third-generation retention cradle that holds and supports the electrolytic capacitor

2. Enhanced PCB location pegs that provide instant mechanical attachment to the PCB while absorbing normal PCB fabrication tolerances

3. Stand-offs, which allow component placement underneath the module

4. Open-ended back end to accept up to a 60mm long capacitor

5. Support/location rib adds strength during impact tests in the capacitors cylinder axis

The final product was then tested, and it passed a battery of automotive level mechanical and environmental tests to confirm compliance to the goals set by our customers. This final qualified product will now become a standard offering from AVX Interconnect.

Tom Anderson is a product manager at AVX. Tom may be reached at tomand@avxus.com.