Hi Dave,
I am not a sales person pushing my wares here, just my engineering experiences - so I hope I am not out of line here on this forum anymore.
When it comes to selective or spot soldering - it all depends on your board design. Many people already have a design and then pick out a machine. This is the difficult way to go, but seems to be the way things are. If so, you need to determine your closest thru hole to surface mount part and find a machine that can reliably perform this process. In our case, we actually designed or own nozzle for our machine. It worked so well that the company that made the machine put it in their standard nozzles to select from.
Now you could always design your board for the machine you like, which is the way to go. Some things to look out for are dross build up. Look for a machine that uses nitro to keep this at a minimum. On this same subject, look out for very small nozzles or static pots. dross build up WILL occur no matter what sales guy tells you otherwise. it will clog the nozzle or static pot, so be ready to have an operator or maint person do daily or shift PMs to keep this clean. Again it depends on your design.
Delta-T is another area of concern. While looking at machines bring your profiler. Top side wetting will be an issue on larger parts. Some machine actually over look pre-heaters (?). Again it will depend on your design.
Flux application is another area to look out for. Selective solder is molten solder. Flux is a spray. You will probably have some flux left on your board that the solder does not get off. Make sure your mood based quality people are ready for this, as it will look different than a wave solder board.
Finally, time is a key factor. Selective solder can be slow. Be ready with some actual times to let your key focused managerial team know - as they think all new equipment is supposed to faster.
There are probably a few other things we could dive into - but without knowing your product, or actually trying to sell you something, I will sit back and await to see what transpires.
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