Solderless assembly of electronic components offers significant advantages in terms of simplicity, repeatability, and many key electrical and mechanical considerations. However, to gain industry acceptance, technologies such as press-fit need to meet stringent operating and performance requirements specified by application-specific and/or standards bodies such as IEC. Press-fit versions of aluminum electrolytic capacitors are now available, providing designers with a useful new device format for a wide range of end applications across multiple industries.
Solderless connection requirements
While most electronic assemblies are soldered using infrared reflow soldering, some components (especially connectors) are not suitable for this method and require different processes to attach them to the PCB. This typically adds additional steps to the manufacturing process, such as wave soldering or hand soldering, increasing complexity and cost. Furthermore, it exposes components already mounted on the PCB to heat shock again, risking performance degradation.
To avoid the cost and complexity of additional soldering processes, we can use a solderless method. In fact, this isn't new—the "blade-and-socket" method has been around for over 50 years. This method involves attaching a secure blade to the component during manufacturing, and then mounting the corresponding socket onto the PCB during reflow. Once the circuit board is assembled, the component can be inserted into a connector to achieve both mechanical and electrical connections.
While this method is very simple, it also has some drawbacks. Because the plug-in socket design needs to withstand multiple insertions and removals, the insertion force is relatively low. This can potentially create resistance and inductance at the joint, significantly impacting the performance of modern high-impedance, high-speed circuits. In some cases, to ensure a good connection and eliminate so-called "fretting"—the gradual degradation of connection performance due to prolonged exposure to vibration—this design requires the use of precious metals.
Crimping provides a better solderless connection
To overcome the shortcomings of traditional plug-in and socket connections, the industry developed crimping technology. In this method, the component is equipped with a compliant pin, and the PCB has a rigid plated through-hole (PTH) directly, allowing the pin to be pressed into it. In effect, the roles of the plug and socket are interchangeable, with the plug (male) applying mechanical force to the PCB hole (female).
Optimized for plug-in sockets, crimping technology offers superior connections compared to plug-in sockets, virtually eliminating resistance and inductance and preventing impact on circuit operation and reliability. Like plug-in sockets, this method significantly reduces additional labor and eliminates the possibility of further damage from heat flow. Furthermore, crimped devices can be plugged and unplugged multiple times if needed, simplifying and enabling risk-free rework or repair.
Furthermore, since crimping eliminates the need for solder to mount the socket on the PCB, it also eliminates issues such as cold spots, solder splatter, bubbles, and cracks, providing highly reliable and repeatable connections. Because components are directly inserted into the PCB, there is no need for precious metals, and the risk of "micro-vibration wear" associated with connector assemblies is also eliminated. The elimination of the socket also leads to reduced material costs.
Challenges in designing crimp terminals
Because crimp devices rely on the interplay of tolerances to ensure a robust and reliable fit with good electrical connections, it's unsurprising that one of the biggest challenges for crimp technology designers is striking the right balance between tolerances. This problem stems from the different manufacturing processes of PCBs and metal crimp leads—PCBs require very tight tolerances, while metal leads require relatively looser ones.
The tolerances for finished through-holes are typically close to the effective characteristics of the pin or the nominal deviation of the "beam," meaning the beam must undergo some plastic deformation during insertion. The design of the beam, including the material and geometry—which determine the insertion force and retention force, respectively—are both critical parameters. Tolerance requirements are: under "tight" conditions, the PTH barrel should not be damaged; under "loose" conditions, the retention force should be sufficient to meet all relevant shock and vibration tests.
Given the high usage volume and low unit cost, capacitor manufacturers must ensure that the design can be replicated with simple machining that meets the required tolerances. Furthermore, the plating used (typically tin) must be as thin as possible to prevent it from being scraped off and forming solder bridges when the leads are inserted.
Crimped electrolytic capacitors
KEMET recently launched its new ALF series of aluminum electrolytic capacitors featuring press-fit leads. This groundbreaking, first-time product line represents a significant step forward for large snap-in electrolytic capacitors.
KEMET uses a proven "eye of a pin" crimping pin designed by Interplex, resulting in a large contact area between the pin and the PTH syringe. Combined with the high lateral force applied to the pin, this enables repeatable, consistent, robust, and reliable low-resistance connections. The pin itself is made of C19010 copper/nickel/silicon alloy—with a tin layer on the nickel plating—ensuring full RoHS compliance.
KEMET's crimping system is proven to handle continuous applied voltages up to 550V, as well as surges and transients up to 620V. From a mechanical perspective, devices using this technology offer vibration resistance comparable to snap-fit devices. This consideration is likely to be crucial as more automotive applications, increasingly challenging industrial applications, and a growing number and diversity of portable electronic devices may face demanding operating conditions.
Crimping devices also help eliminate the need for cleaning PCB assemblies. This cleaning process increases the cost per unit assembly and adds to cycle time and material handling, so anything that helps eliminate this step is desirable.
The use of highly conductive material (50% IACS) on the crimped pins means that the pin resistance is less than 25% of that of typical steel terminals, making this device ideal for continuous high-current applications.
Lower resistance also significantly reduces device temperature rise and helps transfer heat from components to the PCB. In most applications, current limits are defined by PCB traces, not connections. Therefore, these devices offer long lifespan performance even in demanding applications such as industrial, energy/power, and automotive.
Standard ALF capacitors come in cases up to 50mm in diameter; they have one positive and one negative terminal, but also provide additional leads (up to three additional leads, for a total of five leads) to ensure mechanical strength and stability. Higher current versions also provide two internal connection leads for the positive and negative terminals to shunt current loads and provide some redundancy. Furthermore, custom pin layouts can be provided to meet specific customer requirements.
Summarize
For devices that cannot be soldered using infrared reflow soldering, crimping offers numerous advantages, including eliminating the risk of damage from reheating the device. Modern crimped connections have proven to offer very high mechanical strength and electrical performance, making them ideal for demanding applications.
However, not all crimp connections are created equal. KEMET's new ALF series features high-performance pins—resistors that are only 25% of those of steel pins, yet still maintain a strong mechanical bond while meeting the demands of continuous high-current operation.
