Beneath the surfaceThe man with all the answers... Ken Grubb, technical manager of Flint SMD takes time out to answer those frequently asked questions on surface mount connectors. If you have a question for Ken Grubb please contact us. Fax: 0181-652 8956 Email: richard.wilson@rbi.co.uk
Question Can you supply 0.1in. pitch surface mount connectors?
Answer Although 0. l in. pitch connectors are the most popular ranges of PCB through hole connectors, I would not recommend their use for surface mount. Some manufacturers do produce a surface mountable connector, but in most cases the legs have been bent through 90 degrees, and the plastic upgraded to a higher temperature spec. The insertion and withdrawn forces employed on 0 1in. connectors make the reliability of the soldered joint marginal if you are unplugging the unit, especially if the force is not square to the connector.
Question Which style of connector are best for pick and placability using vacuum?
Answer Board to board connectors: these can either be of the "pin and socket" configuration, or the "leaf style" arrangement. When considering a pin and socket style, in most cases there will need to be either a piece of mylar over the contacts or an extra plastic cover that can be used for the vacuum placement. These both add cost to the connector. There can be situations where the product is available on tape and reel, for auto-placement, but the product does not have any facility for vacuum.The leaf style of contact offers flat surfaces on both halves of the connector, although one is situated at the bottom of the connector. This style of contact seems to be the new preferred style as it also offers low insertion and withdrawal forces. This connector design is becoming the norm for pitches of 0.8mm and less.
Question Some surface mount connectors have locating pegs, why?
Answer You would think that with surface mount connectors, the last operation you would consider is having to drill holes. In smaller pitch connectors, it is important that the connector is placed exactly on the pads. In the reflow process a connector, or in fact any component, moves as the solder turns to liquid, and then back to solid. There can be a situation where the connector does not end up central and square to the centrelines of the pads. This can make connection with the mating connector difficult, especially if you have more than two mating connectors on a board. I would suggest that you consider designing in connectors that have location lugs fitted especially when the pitch of the connector is sub 0.8mm.
Question What difference do "hold down tabs" make to the reliability of the connector?
Answer Hold down tabs are small pieces of metal that are moulded into the connector and then are soldered to a pad on the PCB. From experiments conducted with Dr. John Beemish, at the University of Hertfordshire, it was found that the force required to pull the connector from the PCB, increased by 60 per cent when the hold down tabs were fitted.
Question When mating two board to board SM connectors, what X-Y tolerance can the connectors compensate for?
Answer This is difficult to answer because this information is not usually published in a manufacturers specifications. It depends to a certain extent on three main considerations: The pitch of the connector The construction of the connector The style of the contact The 2mm systems are usually of a "pin and socket" type construction, and therefore have good margin on mis-alignment, possibly about 0.5mm accumulative error (x+Y) can be allowed for. Where the construction is of the leaf contacts style, the contact has less margin of error, because the plastic moulding is the main locating force. On the Molex 0.8mm board to board system using leaf contacts it states "when the layout dimensions of one of the PC boards should be within ? 0.3mm to the other PC board in the X,Y direction, then mis-alignment between the housing and the wafer could be absorbed by this connector system." An interesting development by JAE, on their 0.5mm board to board system, is the inclusion of a "floating" connector. The plug connector is fixed, and the mating socket connector can compensate for 0.25mm deviation from either the X or Y direction. This system is available up to 70 circuits.