Press-fit technology

The spring effect achieved by the recess in the middle ensures contact

As a press-fit refers to a special connection technology in the field of printed circuit boards , to establish solderless electrical connections. To do this, a press-fit pin must be pressed into the metallized hole (plated through-hole) of a circuit board. The main feature is that the diagonal of the pin cross-section is larger than the diameter of the hole in the circuit board.

The overpressing that occurs during pressing can be absorbed either by the deformation in the hole or the deformation of the pin. There are two different types of press-fit technology:

Solid pens. This is a non-compressible press-fit contact. The circuit board is deformed in the area of the hole. In this case, the force generated in the area of the circuit board ensures that an electrical connection is established between the sleeve of the circuit board and the press-fit contact.
Flexible or elastic pins with a wide variety of deformation zones. This form of press-fit technique is also known as the needle eye technique. The press-in contact is compressed in the area of the press-in zone. This pre-tensioning of the press-fit contact that is generated ensures that a connection is created between the sleeve of the circuit board and the press-fit contact.

By pressing / cutting the pin edges into the metallization , a gas-tight electrical connection is created, which, when properly executed, is characterized by high reliability and durability.

Materials and workmanship

As post material are brass , bronze and copper - beryllium in question. The surface of these pins can be bare, nickel-plated, tin-plated, gold-plated and palladium-plated.

Nominal thicknesses of 1.5 to 6.4 mm are provided for the circuit boards . The holes in the circuit board should have either uncoated copper > 0.025 mm or copper> 0.025 mm plus tin or tin- lead alloy> 0.015 mm. It is particularly important to maintain close tolerances for the hole diameter. For example, for holes with a diameter of one millimeter for solid pins, tolerances of +0.04 and -0.06 mm are required, for flexible pins +0.09 and -0.06 mm.

The following hole diameters / pin cross-sections are common for square or rectangular solid press-fit contacts:

End hole diameter of the bore 0.65 mm, solid press-in contact, pin 0.5 × 0.5 mm²
End hole diameter of the bore 1.00 mm, solid press-in contact, pin 0.56 × 0.91 mm²
End hole diameter of the bore 1.35 mm, solid press-in contact, pin 1 × 1 mm²
End hole diameter of the bore 1.70 mm, solid press-in contact, pin 0.8 × 1.6 mm²

When processing the press-in contacts, particular care must be taken to ensure that the press-in contact is positioned vertically and centrally in the hole in the circuit board. Manually operated presses or compressed air or hydraulically assisted pressing devices can be used as the press-in device. The press-in process should be technically monitored. In practice, a path-time monitoring or a path-force monitoring has proven to be useful. In addition, a counter-tool for receiving the assembly and a tool insert for receiving the press-fit contact should be precisely matched to the circuit board and the press-fit contact. For this, there must be sufficient free space on the opposite side of the assembly.

As a quality criterion for a press-fit connection, in addition to various long-term tests such as changes in the ohmic volume resistance after temperature changes, moisture storage, temperature storage, storage in an industrial atmosphere, etc., a quick test by determining the push-out force is also suitable. A minimum value of 20 Newtons is common here, but also values of up to 60 Newtons per pen, depending on the application.

history

Press- fit technology was first introduced as a standard in 1984 in DIN 41611-5 “Solder-free electrical connections; Press-fit connections; Terms, requirements, tests ”. The successor document is DIN EN 60352-5 "Solderless connections - Part 5: Press-in connections - General requirements, test methods and application notes".

Historically, the press-fit pins were first used in the early 1970s. In the beginning, the focus was on the mechanical securing of components during assembly and soldering. The Elfab company received a patent for a massive rectangular pen in 1970. Later printed circuit boards were stacked with the help of pressed-in pins and compacted in a multilayer- like construction. In 1974, Winchester Electronics introduced the first elastic pen.

There is now a large variety of different pin geometries from different suppliers on the market. The basic material for the connectors is a square wire that is cut to the required length or thermally torn . Such square wires are used in bare or tin-plated versions.

Today press-fit technology is an established and widespread technology in electronics production.

Advantages of press-fit technology

The following advantages are to be mentioned compared to soldering technology:

no thermal stress on the printed circuit board and the components fitted on the module during the production of the press-fit connection
no solder bridges
no flux residues , no cleaning necessary
Repairability
Environmentally friendly as recycling is possible
The press-fit connections can be made easily.
In the area of the contact point between the press-fit contact and the circuit board, a gas-tight connection is achieved if it is correctly implemented.

Many users today use the advantages of press-fit technology for connectors . By pressing in multi-pole plugs, additional fastening (screws, rivets, ...) has become unnecessary. The areas of back planes, back panels, and motherboards should be specifically mentioned here.

Disadvantages of press-fit technology

Compared to soldered connections, press-fit connections also have corresponding restrictions.

Close coordination of the hole diameter in the circuit board with the press-in contact used.
Consideration of the tighter tolerances for the holes in the circuit boards.
If the circuit board manufacturer or the surface type of the circuit board changes, it is possible that the connection will have to be re-qualified.

The practical use of press-fit connections in large numbers has shown, however, that press-fit technology is a robust technology, provided that the boundary conditions of press-fit technology are taken into account in the design and in series production.

Reliability for press-fit connections and fault patterns

The reliability and service life of press-fit connections essentially depend on the selection and dimensioning of the connection. The following error patterns describe errors that occur during processing or when the module is later operated. Experience has shown that these errors can largely be avoided with correct dimensioning of the connection and correct processing.

Mechanical movement between the press-fit contact and the circuit board: With press-fit connections, it must be ensured that there is no movement between the circuit board and the pin of the component. If movement occurs, for example due to vibrations, there is a risk that the contact resistance between the pin and the circuit board will increase and the connection will fail

Current rating of the press-fit connection: With a soldered connection, the current flow in the hole is distributed over an angle of 360 °. With press-fit technology, the press-fit zones lead to a higher current density locally, since the connection between the press-fit contact and the bore is only made at certain points.

Mismatch between press-fit contact and bore: In the event of a mismatch between the press-fit contact and the bore, the connection can be damaged. If the bore diameter is too large, the contact pressure on the bore is too low and there is no gas-tight connection in the long term. If the bore diameter is too small in the other case, the bore can stretch and the printed circuit board material may be displaced. Both mechanisms can lead to failure of the press-fit connection.

Bent press-fit contact: This error occurs when there is a mismatch between the press-fit contact and the hole diameter of the circuit board. Furthermore, the error can occur if the press-in process is not carried out at right angles to the surface of the circuit board.

Twisted press-fit contact: This case occurs when there is a rotational movement during the press fitting.

Broken press-fit contact: The cause is too high a force on the contact when pressing in or when operating the module or incorrect dimensioning of the hole diameter of the circuit board.

Scraping off the sleeve: This case occurs when there is a mismatch between the diameter of the bore and the press-fit contact during press-fitting. This case also occurs when there is a relative movement between the press-fit contact and the circuit board. Furthermore, the copper or the surface metallization of the sleeve can be scraped off. In the worst case, there may be an electrical connection between neighboring electrical networks.

Sleeve rupture: This case occurs when the hole diameter of the circuit board is too small (see above).

Demolition of the conductor track: If there is a strong mismatch between the press-fit contact and the circuit board, the connected conductor tracks may tear off during the press-fit process. This case is particularly critical if the tear occurs in the inner layers of the circuit board and cannot be found visually.

PCB delamination: If there is a strong mismatch between the press-fit contact and the circuit board, the press-in process can lead to delamination of the circuit board in addition to a sleeve tear in the longitudinal and transverse directions.

Hole diameter in the circuit board too small: If the hole diameters of the circuit board are too small, excessive press-in forces are required when pressing in the contact pins. These forces can damage the circuit board and the holes on the assembly. In this case, the metallized sleeves of the printed circuit board can tear open in the longitudinal direction or tear off completely in the transverse direction. If the bore diameters are significantly too small, there is the case that the press-fit contact cannot be pressed in at all.

Hole diameter in the circuit board too large: If the bore diameters of the circuit board are too large, there is a risk that the electrical connection may temporarily be interrupted due to the insufficient mechanical preload. If the bore diameters are significantly too large, there is a risk that no electrical connection will be made at all.

literature

Andreas Veigel, Michael Tschan and others: Press-fit technology - development, application, qualification . Ed .: Tilman Heinisch. 1st edition. Eugen G. Leuze Verlag, Bad Saulgau 2009, ISBN 978-3-87480-252-9 .

Web links

Michael Tschan: Press-fit technology: What to look out for . In: Electronics . Retrieved March 16, 2017.

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r ^s ^t without author: Harting - Things worth knowing about connection technologies for connectors - Assembly instructions and guidelines for users in electrical engineering - Technical application report 1st edition, Harting Germany GmbH & Co. KG, 2008, p. VIII-1 f.
↑ Solderless electrical connections; Press-fit connections; Terms, requirements, tests . Beuth ( retrieval with costs [accessed December 20, 2012] withdrawn).
↑ Solderless connections - Part 5: Press-fit connections - General requirements, test methods and application notes (IEC 60352-5: 2012); German version EN 60352-5: 2012 . Beuth ( available for a fee [accessed on December 20, 2012]).

[Harting-1] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k ^l ^m ⁿ ^o ^p ^q ^r ^s ^t without author: Harting - Things worth knowing about connection technologies for connectors - Assembly instructions and guidelines for users in electrical engineering - Technical application report 1st edition, Harting Germany GmbH & Co. KG, 2008, p. VIII-1 f.

[2] Solderless electrical connections; Press-fit connections; Terms, requirements, tests . Beuth ( retrieval with costs [accessed December 20, 2012] withdrawn).

[3] Solderless connections - Part 5: Press-fit connections - General requirements, test methods and application notes (IEC 60352-5: 2012); German version EN 60352-5: 2012 . Beuth ( available for a fee [accessed on December 20, 2012]).