Box Type SMD Film Capacitors:
Reliable Solder Quality under Mechanical and Thermal Stress Conditions

Introduction

It is not only the electrical function in the circuit or the observance of particular electric parameters that is important for the functioning of electronic components, but, in addition, the mechanical stability and reliability on the printed circuit board. Especially in the field of SMD technology, the components are subject to much greater mechanical and thermal stress. The following factors are of importance:

Increased Stress Factors for SMD Components

Conventional wired components are subject to much lower stress. The criteria of packing, transport and storing may be considered equivalent, but the stress factors during processing and application differ considerably.

When being assembled, wired components can be held and guided by the terminating wires. Except for possible slight tension on the wires, mechanical stress does not occur on the active part of the component.

In contrast to this, during the assembly process the SMD components are usually handled by pick and place equipment by means of vacuum suction on the component body and placed on the appropriate solder pad on the printed circuit board. The resulting mechanical stress has a direct effect on the component. The component case, or, in the case of non-encapsulated types, the body of the component has to absorb the resulting stress forces.

Whereas in the case of the conventional wired component, the wire had the function of establishing the contact with the printed circuit board / solder joint or with the circuit, as well as fixing and holding the component, the SMD versions are soldered directly via the end contacts onto the appropriate solder pads of the printed board. In addition, the wire was able to take up and absorb the mechanical stress so that it did not reach the active part of the component. Both the solder joint and the component remained, by and large, unaffected by mechanical stress.

With SMD components, however, mechanical stress caused by the effect of outside forces such as vibration, bumps, warping of the circuit board, and also thermo-mechanical pressure or tension caused by differing coefficients of expansion between the printed circuit board and the component, have a direct effect on the component and the solder joint.

In addition, the much higher thermal stress during the soldering process of SMDs due to the integrated heating of the whole circuit board with the assembled components, must be emphasized.

Thus, thermal and mechanical strength, as well as the way the contact is carried out together with the electrical properties, are important criteria for the evaluation of SMD components.