1.1 History

Press in pins have been used on printed circuit boards for over 20 years. At first, oversize pins were used to fix the pins to the PCB and to avoid slippage during the solder process. The development of pressfit technology parallels the advance of printed circuit board technology, from single sided to double sided boards and finally to Multilayer boards. Before Multilayer technology was completely mastered, the stack technique was used, in which several printed circuit boards were separated by an insulation of synthetic film and pinned together by press in posts via drilled through holes. With the development of Multilayer technology this technique was abandoned, leaving only the press in concept. In two decades, press-fit technology has developed over many variations to what is available now: A widely accepted and used solder free technique for connecting a connector pin to a printed circuit board, which is known for high reliability, high contact safety and easy processing.

1.2 Fundamental technology

A press-fit connection between a printed circuit board drilled through hole and a connector pin is made, as the name indicates, by pressing an oversized pin into the drilled through hole of the PCB. The essential feature is that the cross section of the pin must be greater than the diameter of the hole. This results in an overlapping of material at the pin and drilled through hole, which must be taken up by deformation of either the pin or the hole. This gives rise to two different concepts for press in zones:

• a solid pin which will not deform
• a compliant pin which will deform

Figure one depicts a cross section of a compliant and a solid pin.

Of the two methods, the compliant pin technique is the preferred for making press-fit connections. The solid zone was mainly used in the above mentioned multiboard technique for electrical and mechanical connection of printed circuit boards on top of each other. The compliant pin press-fit method has several advantages over the solid pin technique:

• Because the press-fit zone of the pin compresses, the plating of the drilled through hole is saved from possible destruction of the through connection to the circuit paths.
• The compliant pin allows a bigger tolerance of through plated holes.
• Reduction of insertion force.
• Multiple pressing of contacts into the same hole is possible.

1.3 A comparison of the solder versus press in method

Press-fit technology has a number of advantages over soldering:

• Eliminating soldering eliminates thermal stress of the printed circuit board.
• No solder lugs or flux residue which may impair the contact of the connector.
• No cold-iron soldered joints.
• No short circuits by soldered connections.
• Attachment of connector directly above press in pin (no screw mounting necessary).
• Longer wire wrap posts remain free of solder tinning and can be used as male connectors on the other side of the board.
• Defined impedance of the connection (good high frequency characteristics).
• High speed, low cost assembly of the connectors.
• Full repair capability - connectors and contacts can be easily exchanged.

An essential but often underestimated advantage of press-fit technology versus soldering is the economic advantage of assembly to the printed circuit board. Studies by various firms have shown that the extra costs generated by solder problems are widely underestimated. In one confirmed calculation of the economics of the two processes the press-fit technique is shown to be much more economical in comparison to the standard solder technique.

1.4 Applications

Press-fit technology with compliant press in zone is today's state of the art technology with Bus backpanels of all kinds. Because of the requirement for high connections on the backpanel, and the need for plug-in capacity on the backside, press-fit has become the method of choice in a wide variety of applications. At present, there are few gain measuring devices, Multibus II or other backpanels made with the soldering technique.

A very interesting field for press in connectors is opening up in PCB component assemblies. The increasing use of SMD components and of the reflow soldering process makes soldering of the connectors a big problem (presently there are only a few SMD connectors on the market). Here press-fit technology can be a solution, in that during reflow soldering, the drilled through holes remain open, and thus the connectors can be assembled as a final operation. This can be particularly advantageous in boards with components on two sides.

HKP 35 tool kits