Are Sockets and PCB Connectors Technical Twins?

Autor / Redakteur: Gerhard Brüser * / Kristin Rinortner

Sockets and PCB connectors have many similarities. For example, the contacts are very similar and the soldering methods do not differ either. The article highlights differences and similarities.

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Figure 1: Standard DIL-IC sockets and plugs in variable form and Sockets for IC-PLCC.
Figure 1: Standard DIL-IC sockets and plugs in variable form and Sockets for IC-PLCC.
(Source: © Fischer Elektronik)

At a first glance, sockets and PCB connectors appear to have absolutely no direct connection to an outside observer. Quite different technical applications can be derived from them. However, on closer inspection, they have a lot of characteristics in common and are therefore sometimes very similar in terms of detail. Sockets are used to hold electronic components – circuits – in a wide variety of designs and sizes with a great variety of connections.

In the event of damage, a technician should be able to change the components quickly and easily, without stressing the circuit boards with unnecessary soldering work. Printed circuit board connectors are more likely to be used to connect printed circuit boards to one another, to power the assemblies and, last but not least, to transfer data. Designs with similar detailed structures should be considered in this instance.

Some Aspects of the Development of Sockets

In the traditional sense, the function of a socket is that of connecting the connecting pins of an electronic component, for exam­ple IC (integrated circuit), to "grasp" the contact safely and to make a reliable contact. These "integrated circuits" were very expensive components at the beginning of electronic development in the 1980s.

In order to be able to replace them and to avoid soldering them onto the circuit board, appropriate "sockets" have been developed. The many different designs of IC components and their different connection contacts have resulted in just as many socket types. In addition to the grid contact to contact, the row-row spacing must be respected.

The most common version found is the DIL version, the “dual-in-line” version, for DIL ICs. In addition, there are round sockets with different numbers of poles for transistors, more specifically designed for power transistors, for quartz oscillators and also sockets with plastics that are particularly resistant to high temperatures.

There are also other types of frames, e.g. for PLCC-ICs (Plastic-Leaded-Chip-Carrier) that are still in use today. These IC modules have a J-shaped connection form which, in conjunction with the corresponding fitting, forms a clamping and spring contact.

Relation of Sockets to the PCB Connector

The structure of the socket headers is similar or even partially identical to that of the sockets with regard to individual contacts. When selecting precision contact types, the same socket contacts can be used as with the sockets. The only requirement is that the plug-in cross-sections of the pins are matched to the socket contacts.

Figure 2: 
Special sockets with different number of terminals, diverse housings form and with precision and stamping contacts.
Figure 2: 
Special sockets with different number of terminals, diverse housings form and with precision and stamping contacts.
(Source: © Fischer Elektronik)

The connection pins of IC modules are suit­able for a size corresponding to 0.5 mm round pins. Therefore, the mating strip – the pin contact strip – is then usually designed on the plug-in side with round connections with a diameter of 0.5 mm. In the early days, the development of the printed circuit board connectors began mainly on the basis of precision contacts, similar to those of IC sockets. They are mainly used in single and double row strips. In addition, there are also some variants in three-row and even four-row designs.

A more cost-effective way to use multi-row strips is to design the single and double-row strips so that they can be joined together.

For larger contact cross-sections, such as the common square pin that has contacts with an edge dimension of 0.635mm, obviously, the socket contacts have to be larger to match the corresponding cross-section. In many cases, the inner spring is also selected with six contact fingers instead of four contact fingers for the purpose of even more secure contact.

Precision socket contacts are made up of two parts called the sleeve and the inner contact spring. The sleeve is rotated while the inner spring is punched and rolled. After the galvanic coating, it is inserted into the sleeve and fitted with a press fit. The inner contact spring, which is also known as a “clip”, can have a different number of contact fingers, from three to six “fingers”, depending on the size and the desired contact reliability. The contacts for the IC sockets usually have four contact fingers. Tin-plated and gold-plated versions are offered as coatings, with the gold-plated versions being preferred. The sleeve, on the other hand, is mostly tin- plated.

Figure 3: 
PCB male and female connectors with precision and stamping contacts, in THT- and SMD-soldering versions. The surface can be in gold or tin.
Figure 3: 
PCB male and female connectors with precision and stamping contacts, in THT- and SMD-soldering versions. The surface can be in gold or tin.
(Source: © Fischer Elektronik)

The type of punched flat contact sockets have not become popular, in contrast to the popularity of the PCB connectors. Die-cut contacts are by far the most popular in this instance. Those known as “tulip-shaped” contacts with two flat contacts or fork contact shapes are used frequently.

These types of contacts are very easy to assemble automatically. In addition, the galvanic coating, specifically gold plating, is possible very well adapted to strip electroplating, making a saving on precious metals. This creates a good and cheaper alternative to precision contacts compared to precicion contacts. In the pin header area you will find a predominance of square material drawn. The tips are shaped like a pyramid to enable a decent placement.

Soldering Process and Plastic Selection

Soldering processes are used for sockets and PCB connector correction. This involves the wave soldering process, various “reflow” soldering processes and includes vapor phase soldering that is gentle on components. For the insulating body material special high-temperature quality and applications have been developed in recent years.

However, the demands on the connectors and sockets have increased significantly due to the high soldering temperature during SMT soldering. With the wave soldering process, the connectors are barely exposed to the usual maximum continuous temperature load, while with the SMT soldering techniques temperatures of approximately 260°C are common. It is assumed that the insulat­ing bodies are made from appropriately high-temperature-resistant plastic.

For smaller manufacturers of connectors and sockets it is hardly worth their while to use different plastics for the wave soldering process and the SMT soldering process with the same design of the insulating body. In addition to the possible risk of confusion if you choose the same plastic color, double storage is also an issue that should not be neglected. It is therefore advisable to always use the high-temperature-resistant plastic for all identical designs, regardless of the soldering process.

On top of that, for particularly thin-walled components, only high-tech plastics such as from the LCP group (liquid crystalline polymers) can be used. For this, however, corres­pondingly suitable molding tools are re­quired which, due to their thin-bodied plastic, have to be perfectly matched, completely form-fitting and manufactured with high precision.

The temperature control of the molds, at over 130°C, must also be respected for both machine technology and tool technology. This means that even the smallest wall thicknesses can be filled reliably, even with a very long flow path. This is achieved through the molecular structure, with rigid, rod-shaped macromolecules that align themselves in parallel in the melt.

As a rule, the LCP materials are inherently flame-retardant, level V0, in other words, they are not affected by flame-retardant additives. By adding fillers such as fiber or mineral fillings, a highly dimensionally stable plastic is created.

This means that extremely fine structures, which arise with many connector strips, especially when the grids are getting smaller, can be safely filled and produced. A reason­able heat resistance exceeds 270°C and, for some types, can even reach over 300°C. In addition, quality mechanical properties are achieved with this material.

The electrical characteristics are important features when selecting sockets and PCB connectors. The common types with a contact spacing (grid) of 2.54mm are usually at a nominal current load of up to 3A.

Electrical Requirements for Selection

The current carrying capacity is reduced with smaller grids, e.g. 2.0mm and 1.27mm, to current values of 2.5A and 1.0A depending on the grid and type. There are also possibilities for variation through the use of different contact materials. While the electrical conductivity of tin-bronze (CuSn) is around 9 S / m, brass (CuZn) has around 15 S / m.

If good spring properties are required, tin-bronze alloys are more suitable. In most cases, these options for electrical load capacity are completely sufficient for the mass market of PCB connectors. In addition, materials made from CuBe-Alloys with up to max. 2% beryllium were established. In addition to variants in the mill-hard state, hardenable types are also available. The curable variants can be used especially for strongly shaped spring elements.

* Gerhard Brüser is the lead development engineer for connectors at Fischer Elektronik in Lüdenscheid.