You may have noticed that some board mount connectors have extra pins beyond the contacts that are listed in the part’s description.
Hirose part numbers FX4C3-80P-1.27DSAL(71) and HIF6B-80PA-1.27DSAL(71) are just two examples of this.
Hirose refers to these pins as ‘lock pins’ or ‘board prefixed pins’. Other manufacturers may call them ‘mechanical hold downs’ or something similar.
The addition of these pins addresses the need for mechanical robustness. They add strength and stability to the connector to increase it’s reliability when faced with vibration or multiple mating and unmating processes.
The hold down pins (or solder tabs) are particularly useful on surface mount components. As connectors reduce in size and move towards more surface mount designs, they become less robust. Contacts are smaller and thinner and are more easily bent because these connectors don’t benefit from the secure attachment to the board that can be achieved with traditional through-hole solder contacts. Through hole connectors can also have additional mechanical hold downs, such as screws or board locks, which would not be practical with surface mount connectors. When considering factors such as vibration or repeated connecting and disconnecting (both of which can create stress on solder joints), a secure connection to the board is essential.
Through-hole plastic guiding pins help alignment and also give additional strength and stability against force.