Circuit board connectors Select connectors based on the application

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There are several ways to achieve a high-quality connector connection. Some can be achieved by correctly selecting the individual components, others depend on the manufacturing quality of the manufacturer. Read here to find out how to choose the right connectors for your application.

Timon Dahlhaus is a development engineer for PCB connectors at Fischer Elektronik.

For the design of a high-quality connector on the circuit board, the developer must consider various criteria. These include the choice of materials, the soldering process, and the correct pairing of contact pin and socket. In addition, the different versions of the pin and socket strips play an important role for the respective application.

These two types of connectors can be divided into different quality levels. The use of connectors of each quality level is legitimate and can achieve the required performance in various application areas. The following sections describe the various quality levels and explain why even less good and cheaper connectors can provide high-quality connections.

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Material selection: The insulator body

The quality of the insulating body in PCB connectors is largely determined by the technical thermoplastic used. A high fiber glass content in the plastic improves the shape stability, dimensional accuracy and resistance to mechanical loads. Although technical thermoplastics may seem overdimensioned for applications without high temperatures, they are essential due to the high temperatures in the reflow soldering process and the good flame retardant class. In the end, it is by no means advisable to make qualitative compromises when selecting the material for the insulating body. Here, attention should always be paid to a UL94 V-0 certification and the best mechanical properties.

The right contact material combined with the right coating

In electronics, copper alloys, which are characterized by high electrical conductivity, are typically used for contacts of pin or socket strips. Brass and bronze are the most common materials, while beryllium copper, for example, is less commonly used, but is ideal for precision inner contact springs. However, these alloys tend to react with oxygen and form a passive layer, which affects electrical conductivity and increases the risk of short circuits due to flaking particles. To avoid this effect, a finishing layer is required.

A gold finish offers excellent conductivity and, due to the hardness of the material, a high number of mating cycles. Thus, gold is an optimal surface coating, with the thickness of the gold layer being crucial for the lifespan and resistance of the connector. Even thin layers of just 0.2 µm can guarantee 50 mating cycles, while a thinner flash gold plating of 0.02 µm can guarantee a maximum of ten mating cycles. In applications where the connection is rarely or never disconnected, the thinner gold layer provides a sufficient and resource-saving solution.

Tin - the alternative to gold

Tin is a more cost-effective alternative to gold on contacts. Tin is especially suitable for applications with low currents up to 3 A and offers good conductivity for up to ten mating cycles. Furthermore, tin has better solderability, regardless of whether the connector is soldered using wave soldering or a reflow soldering process. The use of selectively gold-plated contacts combines the advantages of both materials: the gold-plated side ensures a low contact resistance, while the tin-plated side ensures optimal solderability (image 1).

A silver-plated surface even surpasses a gold coating in terms of electrical conductivity. Silver offers the highest conductivity of all materials, but is also prone to the formation of an oxidation layer, which leads to contact difficulties with repeated plugging operations. Therefore, the exact application and the associated requirements, such as the number of plugging operations due to (de-) assembly, must be taken into account when choosing the contact material.

The differences in contact springs

The connector specialists in Lüdenscheid use two types of contact springs in the socket strips: a stamped fork contact and a round, stamped-coiled contact spring. Which contact spring fits best depends on the area of application and the environmental conditions.

The stamped-coiled contact spring is suitable for environments where the electronics are exposed to vibrations and shock loads, as its six contact fingers ensure reliable contact (Image 2). Even strong movements are compensated in this way without interrupting the connection to the contact pin.

In contrast, the fork spring with a rectangular cross-section is a cost-effective solution for static applications. The fork spring touches the contact on two opposite sides. In combination with a square contact pin, the contact surfaces fit perfectly together, maximizing the contact area and achieving an effective electrical connection.

SMT and THT: The connection technology - circuit board connectors

Once the appropriate parameters for the quality level of the pin and socket strip pair have been selected, the last step is to choose an adequate connection technology between the connector and the circuit board. The Surface Mount Technology (SMT) is particularly recommended when the connectors are assembled using a fully automatic system.

In this process, the component is sucked up using a suction device on the assembly aid or a flat surface and placed on a pre-assembled solder pad. Only in the reflow oven does the solder pad melt and establish a secure connection with the connector.

The Through-Hole Technology (THT) is characterized by increased stability of the soldering. The combination of the hole in the circuit board and the contact offers additional robustness to the soldering. The disadvantage is that automation is more difficult in the soldering process, as it doesn't occur in the reflow soldering oven, but with a conventional soldering wave.

The automated placement of the connectors is made more difficult by the additional fit between the hole in the board and the contact pin. The positioning tolerance is significantly smaller than is the case with SMT.

THR: The fusion of SMT and THT

The Through-Hole-Reflow technique (THR) has emerged from the fusion of the two connection techniques mentioned above. This combines the advantages of both soldering processes and guarantees a stable and modern connection between the connector and the circuit board (Image 3). This results in a stable and secure connection that offers stability by using the holes in the circuit board and saves space in the circuit board layout.

Due to the Pin-In-Paste technique (PiP), the connector can be soldered in the reflow oven automatically. The only disadvantage compared to SMT is the complicated positioning. However, this is also possible with modern placement machines and increasingly precise connectors, making it more and more popular.

In addition, there are applications where soldering is not possible. This could be due, for example, to another component on the circuit board that would be damaged by the temperature of the peak zone of the reflow oven.

Pressfit: When soldering is not possible

For these cases, there is also the option to use press-fit connectors. These are simply pressed into the hole in the board and do not require any additional soldering. A punched area in the contact pin is responsible for this, which deforms when pressed into the board in such a way that this connection can only be loosened again with a high effort.

Conclusion: There are several ways to achieve a high-quality connector. Some can be achieved by correctly choosing the individual components, others depend on the manufacturing quality of the manufacturer. If you consider the mentioned factors and specifications, nothing stands in the way of a long-lasting and durable connector. (kr)