“Thin is In” for Microminiature Board-to-Board Connectors! Or is it?
The trend in recent years for microminiature board-to-board (B-to-B) connectors has been toward lower profiles. The most commonly used B-to-B pitch in handheld consumer electronics devices is 0.40mm; and while 1.00mm stack height used to be considered low-profile, the most recent connectors being used by major OEMs are as low as 0.70mm, and sometimes even lower. As we get closer to the limits of what is physically possible with traditional B-to-B design, other factors are now taking on more importance, and one of those is connector width.
Older microminiature connectors with a width of more than 4.00mm were replaced by connectors in the 3.40mm width range some years ago, and the current definition of “Narrow Body B-to-B” is typically in the range of 2.40 to 2.60mm. But is narrower really always better? Here are a few things to take into consideration in your B-to-B connector selection process, in addition to the basic questions of circuit size, stack height and width:
- Do you have freedom to run traces underneath the connectors? If not, you should look for a receptacle design that uses top-mounted terminals.
- Do you need to apply conformal coating to the solder tails? If so, then you will again have to avoid receptacles with bottom-mounted terminals, and you will likely need a connector with solder tails, which protrude a certain distance from the housing.
- Are you willing and able to develop a custom pick-and-place nozzle for your selected B-to-B system? If not, then you should look for a connector that is available with a pick-and-place tape or a cap.
- If you are able to invest in a custom nozzle, can you pick up around the perimeter of the connector, which will likely involve some loss of suction? Or, must you pick up on a solid, flat surface, which will typically be the bottom housing wall of the plug or the central wall of the receptacle?
- What is your minimum width of pick-up area on both the plug and receptacle (they will likely be different!) in order for your vacuum nozzle to handle the connector efficiently? Have you taken into consideration the movement of the connector in the tape-and-reel pocket, and the accuracy of each of your SMT machines?
- If the manufacturing of your product will be at one of your EMS partners rather than in-house, are you familiar with the requirements of their SMT lines?
- What kind of unmating forces are you looking for to ensure secure and reliable mating during drop-testing? You may be planning to use your device mechanics to apply pressure to the mated B-to-B connector pair, particularly if you are using a small circuit size connector that does not generate sufficient unmating force. But Molex also offers newer B-to-B systems which provide much higher unmating forces, which may allow you to achieve sufficient unmating force even without any help from your device mechanics.
Molex has many different 0.40mm pitch B-to-B connector systems, which were all designed to address some specific requirements. But there really is no “one size fits all” B-to-B system which will solve every potential problem, and you should think through your specific requirements and all the available options before you make your final choice rather than jumping to quick conclusions. Some points to consider:
- Surely narrower is better, right? Well, it may be, if you are willing to accept a narrower pick-up area for your vacuum nozzle. Before selecting a super narrow connector, you should check whether it provides all the other features that you are looking for, and test it thoroughly to ensure that it provides the degree of robustness that you are looking for.
- OK then, but it looks like I should select a receptacle with top-mounted contacts, because that will allow me to run traces underneath the receptacle, and probably allow me to apply conformal coating, right? Well, some of the downsides of receptacles with top-mounted contacts are that the width of the vacuum pick-up area tends to be narrower, there is no plastic housing material above the receptacle terminals to prevent them from lifting during very rough unmating, and the top surface of the receptacle is more uneven, which can make it more difficult for operators to mate the connectors.
- These newer connectors with higher unmating forces sound great! Yes, but you may need to allow for some extra connector length for the locking features these connectors use; and higher unmating forces usually also mean higher mating forces, which may become problematic for high circuit sizes.
There are many trade-offs like these, which you need to consider thoroughly in your B-to-B selection process. You should try to decide which are the features where you absolutely cannot compromise, and which are the features where you perhaps have some degree of flexibility. Your local Molex team will then be able to help you select the most appropriate connector for your particular application and requirements.