How to Select Connectors for Hardware Design

The design of electronic products tends to be more and more modular. Through the organic combination of different functional modules, high-quality products with different properties can be quickly created, and the connection of modules is particularly important. Here are some experiences about connector selection shared by a senior hardware engineer.

Just like the functional interface of the program, if the design of connectors is reasonable, the future maintenance, upgrade and transplant of products will get twofold results with half the effort, so that the enduring product service life can be kept; The unreasonable design will make future maintenance and upgrade difficult. As a saying goes: A slight move in one part may affect the situation as a whole. Finally, the product will lose the competitiveness. Therefore, the importance of connectors is self-evident.

The connectors are commonly known by engineers as connector assemblies, which are used to connect two circuit boards or electronic devices for the transmission of power or signals. Connectors can modularize the circuit, simplify the assembly process of electronic products and make the products easy to maintain and upgrade.

There are many types of connectors, such as communication interface terminals, wiring terminals, wire-to-board connectors and board-to-board connectors. Each category can be subdivided into several categories. For example, board-to-board connectors include pin headers and female headers, PC104, board-to-board connectors, etc. Wire-to-board connectors include FPC connectors, IDC sockets, simple horn sockets, etc. The selection of connectors plays a crucial role for modular circuits, so what angles should we consider in the selection of suitable connectors for the hardware?

1. The pin and spacing

Pin number and pin spacing are basic bases in the selection of connectors. The selection of pin number depends on the quantity of signals to be connected. For some patch connectors such as patch pin headers shown below, the pin number should not be too large, because the connector plastics will deform and swell in the middle part due to the high temperature effect in the welding process of the chip mounter, which will result in the insufficient soldering of pins. In the early study and development stage, our P800Flash programmer used this pin header and female header to connect the boards. As a result, the insufficient pin soldering of prototype pin headers occurred in large area, and the problem never arises again after replacing the original pin header and female header with two pin headers whose pins are halved.

Nowadays, electronic devices are developing towards miniaturization and precision, and the pin spacing of connectors is also reduced from 2.54mm to 1.27mm and then to 0.5mm. The smaller the pin spacing is, the higher the production process requirements are. Pin spacing should be determined by the production technology level of the company, or the blind pursuit of small spacing will cause difficulties in production and maintenance.

2. Electrical properties

The electrical properties of the connector mainly include limited current, contact resistance, insulation resistance and electrical strength, etc. Pay attention to the limited current of the connector when connecting high-power power supplies; pay attention to the contact resistance when transmitting high frequency signals such as LVDS and PCIe. Connectors should have low and constant contact resistance, which is usually dozens of Ohms to hundreds of Ohms.

3. Environmental performance

The mechanical properties of the connector include insertion and extraction force, mechanical fool-proofing property, etc. The mechanical fool-proofing property is very important to the connector. Once it is inserted in reverse, irreversible damage to the circuit will be caused!

The insertion and extraction force can be divided into insertion force and separating force. The relevant standards stipulate the maximum insertion force and the minimum separating force. From the perspective of use, the insertion force should be small, while the separating force should be large. Too little separating force will reduce the reliability of contact. However, for connectors that often need to be plugged or unplugged, too large separating force will increase the difficulty of pulling out and reduce the mechanical life.