Today, PCBs should be equipped with all the necessary active and passive components on both sides. This is particularly important for applications where only limited installation space is available. This is where surface-mounted technology offers a solution: It uses tiny components that are not fitted with pins, but are attached directly to the PCB. This eliminates the need to drill holes for mounting the components. This makes PCB design much more flexible for developers, as it also enables additional components to be fitted to the rear of the PCB. This makes SMT equally interesting for the production of smaller assemblies or as a solution for specific applications.
In addition, the range of substrates that can be used is growing - for example, glass substrates that cannot be drilled can also be used. Instead, the conductor tracks are vapor-deposited onto this base surface.

Restrictions become obsolete

However, the use of SMD (surface mounted device) components sometimes reached its limits: This affected connectors above a certain size and a pitch of more than 2.54 mm. Until now, there was no way around through-hole mounting to attach the components to the PCB. The reason for this is that the conductor connection and the power supply need sufficient dimensions to meet the physical requirements for higher currents and voltages.
PCB terminals are also exposed to greater mechanical loads than other passive or active electronic components. Enormous forces are generated during the assembly process - whether by connecting electrical conductors or by attaching a corresponding connector strip. The adhesive forces of the terminal block do not always withstand the installation requirements, and components often come loose from the circuit board.

Floating elements

In contrast to fixed contact elements, Weco's floating pin technology offers 100% coplanarity, optimum soldering, minimal contact resistance and maximum strength.

© Weco

In order to get to grips with this problem and bring an adequate solution to the market, Weco spent years conducting basic research. In the course of this work, it emerged that a reliable holding force of the terminal can only be guaranteed if the solder joints make reliable contact with the PCB. This applies to all solder joints without exception. However, this becomes a problem with larger components or large-pole connection terminals.

A solution to this problem has been developed with so-called 'floating contact elements'. Depending on their design, these contact elements can move freely in all directions and reliably touch down on the PCB surface or solder joint. The core of the technology is to achieve a minimal fixation of the axis while at the same time guiding the contact element. This achieves mobility within the tolerances in the predefined axes. Neither the size of the components nor the number of poles have any influence on the end result.
The '930-D- SMD-DS' terminal block with a 3.5 mm pitch from Weco, which is suitable for a conductor cross-section of 1 mm², is a good example of what this means in detail. The terminal body is movable in the housing. Movement in the horizontal axis is achieved by a precise fit between the metal component and the housing wall. In the vertical axis, a predefined height variability in the housing has been designed in order to achieve a secure and coplanar solder connection between the contact element and the PCB.

The main advantage over a classic THR solution or through-hole reflow soldering lies in the flexible, active function of the contact element. Due to the weight of each individual element, an ideal position is achieved in the form of coplanar positioning and therefore the perfect solder joint with the lowest contact resistance. This optimum connection between contact element and PCB results in maximum strength. Tear-off and pull-off forces have almost the same values that are appreciated in through-hole technology.

In many years of tests and endurance tests as well as tensile tests in our in-house laboratory, 50 % higher retention forces were determined with the Weco floating principle compared to the screw torque. The tensile tests on the two-pole PCB terminal resulted in holding forces of 100 N. Due to these high holding forces, additional lateral soldering flanges to enlarge the soldering surface are not necessary - for users this means additional space savings on the PCB.