Through-hole technology for the reflow ovens

In this informative article, Schurter delves into the dilemmas between through-hole and surface-mount technology and the history of solutions. Electrical and electronic components come in various sizes and mounting technologies. The classic is through-hole technology (THT); its modern counterpart is surface-mounting technology (SMT). Unfortunately, these two technologies, combined in almost every electronic device, need different soldering methods. A dilemma? Yes and no. 

At the beginning of the 20th century, no PCBs existed. All components available at that time were wired freely by hand. It was not until around 1920 that the first prototypes were created: stamped conductor tracks riveted to hard paper and held together with sheet metal springs. In 1943, the Viennese engineer Paul Eisler was granted a patent for a PCB, with moderate success for quite a long time. Manual wiring remained the standard for a good decade. 

Through Hole Technology (THT) It was in the early 1950s that the PCB slowly gained acceptance. Near Düsseldorf, the through hole technology was born in the Ruwel-Werke. The connecting wires of the components were inserted through drilled holes in the PCB, which were provided with copper conductor tracks on their underside. This approach simplified production and, at the same time, reduced the error rate during wiring. Today, this is called THT: Through Hole Technology. 

Surface-mounting technology (SMT) is not much younger, even though it is used for almost all modern electronic products. Its beginnings can be found in the 1960s, developed by IBM for the computers of the Saturn and Apollo missions. The reasons for this development at the time were the cramped space conditions in the spaceships and a reduced circuit impedance to increase switching frequencies. 

Miniaturization SMT and THT have a fixed place in the production of every EMS company today. EMS stands for "Electronics Manufacturing Services", i.e. the production and assembly of complete assemblies. The increasing customer demand for mobile electronic devices is shifting the focus more and more to surface mount technology. 

Usually, SMT components are much smaller and thus allow more compact end devices. Smartphones are the best example of this. Without SMT, they would be unthinkable in their current form. In contrast to through-hole mounting, SMT components are "glued" directly onto the copper-clad surface of the board and then soldered in a reflow oven. An SMT PCB often allows assembly on both sides, doubling the possible, fully automated assembly density. 

Hybrids as a consequence However, only some components can be reduced in size at will. Stationary electronic devices almost always have a built-in power supply. Traditionally, this consists of a transformer, capacitors, resistors and a rectifier. However, even the switching power supplies used frequently today cannot be "shrunk" to miniature dimensions, depending on the power required. Power needs space. If, for example, the power supply also has to be placed on the SMT circuit board, then it quickly becomes a tight squeeze for a transformer. Or let us consider the question of fuse protection: If a fuse blows in the event of an overcurrent, it would be extremely useful if this fuse could be replaced without great effort. This need led to hybrids: SMT circuit boards containing further drill holes for THT components. 

The consequences of a hybrid The use of two technologies has consequences in the soldering process. For the EMS provider, each board must undergo two soldering processes. One for the surface-mounted components (reflow method) and a second for the components in through-hole assembly (wave soldering. Two soldering processes are associated with significantly higher costs and a longer production time. Also, two soldering systems must be available. But there are other disadvantages as well. 

Issue: Ageing If a hybrid PCB has to go through two soldering processes, many components are heated twice to temperatures well above 200C. This is not beneficial to them, and high temperatures will shorten the life of any electronic component. Issue: misplacement The double soldering process poses an additional risk from its practical implementation: It is usually the case that the THT components are inserted after the reflow soldering process for SMT. In particular, manual placement of the components for the second soldering cycle in the wave bath involves a massively increased risk of incorrect placement. 

Approach: no hybrids To avoid these problems, there are several approaches. The simplest is to prevent them from occurring altogether. In other words, use only SMT or only THT components. Then a single soldering process is always sufficient. However, this is often not possible in practice due to the technical properties that the end product to be soldered should have. 

Alternative: THR The abbreviation THR stands for "Through Hole Reflow". THRs are components with through-hole technology. However, these THR components are designed for automated assembly and high thermal stress in the reflow oven. During the assembly process, a paste is first printed in the vias for the THT pins, and then the component is pushed through the solder paste. As the paste melts in the reflow oven, the liquid solder retracts into the vias due to wetting and capillary forces, forming a clean solder joint. Two technologies, one soldering process. Highly efficient! 

Back to (fuse) protection With this in mind, we should look again at the circuit protection of a fully automatically assembled PCB. It would be highly advantageous to install a fuse holder on the SMT board, which can be soldered immediately in the reflow process. Such fuse holders exist. The glow wire-resistant, open fuse holder Schurter OGN is a classic, and it is designed for 5x20 fuses of various rated currents and tripping characteristics. It can also be readily converted into a closed fuse holder using a cover if desired. Three versions are now available: classic THT, SMT and THR, which is fully compatible with the THT version. The right solution for almost every application. One soldering process is enough.