Distribution world – Design tips for choosing surfae mount connectors

Distribution world – Design tips for choosing surfae mount connectorsKen Grubb
Time to market pressures inevitably dictate a quick learning curve – made steeper by the almost universal tendency to design the connectors last! If you are scrambling up this learning curve for surface mount connectors here are one or two things to consider. Assembly issues
For a start, there are some basic questions the designer needs to put to the production director, before even thinking about which connectors to use. The first is, how fine a pitch can our pick and place machine handle? Some fantastic space saving fine pitch connector families are available on the market, down to 0.5mm and even 0.3mm, but there is no point in specifying them if your machine is not be capable of putting such pitch down.
The second question should relate to the machine’s capacity in terms of coping with wide component reels. Manufacturers have got the point about supplying SMT connectors on reels now, but the large size of some components dictates that connector reel widths are larger than you commonly see. Chip passives reels are usually 7mm, some connector parts can be as large as 56mm width. Check you have the applicator for these available.
In the UK, the standard placement method is vacuum as with normal components but some, especially Japanese, connectors are only suitable for robotic assembly. This is a particular issue with vertical wire-board and board-board headers. Check that the connector you choose has the capability of being picked up by vacuum. Ask your supplier for a strip of the tape with the connectors to try in your production environment! Design constraints
The next issue to consider is the actual design of the SMT connector. There are still some manufacturers peddling through hole designs with their legs bent, but these solutions cannot match the best surface mount only designs in pitch, for a start. The best type are the fine pitch, 1.5mm or less, which have been specifically designed for SMT.
Amongst the features to look for are PCB contacts that sit firmly against the connector body, so that they cannot flex. Fine pitch connectors usually have “hold down tabs” which have been shown to greatly improve the board retention of the product.
There are some products that are now becoming “commodity” type connectors (for example flat flex connectors), with more than one manufacturer supplying the SMT market. Why not put down dual footprints to take two sources of product?
Note that, with flat flex cable (FFC) connectors the price of connection either with top or bottom contacts in the same, but with flat flex cables if you have the contacts on either side of the cable there would be up to 30% increase on cable where the connections are on the same side.
Wire to board connectors there has been a level of standardisation on 1.25mm pitch product. This style is available from at least 3 or 4 different manufacturers but most are not fit and function compatible with each other.
Most of these ranges of wire to board do not have hand tools available, and if they do, with crimps only 3.0mm x 1.3mm diameter, reliable, repeatable crimps could be a problem. Machine crimping is by far the best option but for reliability make sure that your supplier can make small quantities for you to test.
The diversity of connector products that is available in surface mount is expanding all the time with DC Jacks, Mini DIN, RJ45, RJ11 all on offer in true SMT fitment and in tape and reel. One final point, always check whether the part is in production and what are the minimum order quantities. If your chosen connector fits all of these constraints, then you’re likely to be in good shape.
Ken Grubb is marketing manager for connectors at Flint Distribution


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