Do any SMT components require hand soldering due to heat limitations?

Question

It has been claimed on another board that some SMT components cannot be used in an automated production line (i.e. reflow or wave soldering) because they cannot withstand the heat/temperature. Hand soldering is used to avoid these problems.

An example given where this was the case was film capacitors. Capacitors are often the component which limits the maximum temperature and time at high temperature, but I haven't seen any that preclude the use of reflow soldering.

I haven't encountered a single SMT component that cannot be used in an automated soldering process in the past. Some require care i.e. the temperature profile must be carefully chosen and times at high temperature limited. Some components do not support the use of lead free solder (in terms of maximum temperature), but these are becoming rare.

I suspect that requiring hand soldering is either due to:

• Limitations in the equipment i.e. the reflow oven cannot deliver heat quickly enough to meet the requirements.
• Poor design i.e. it hasn't been designed with reflow in mind and large and small components cannot be soldered at the same time.
• A few very special components require special treatment and it is easier to hand solder than change manufacturing processes.
• Small scale production where hand soldering doesn't cause costs to explode.

I also feel it might be incorrect to assume that hand soldering results in lower temperatures, but that is not the core question here.

I don't think it would ever be an inherent limit in an SMT component - they pretty much came about to support automated manufacture.

Thoughts?

Answer 1

I have never encountered a SMD part that needed to be hand soldered.

Probably what someone ran into is a part that needed a special profile such that the board would have had to be run thru the oven twice. If this is due to a single part, it could well have been more cost effective to run the board thru the normal process and hand solder the special part afterwards. It's not that it couldn't be reflow soldered, but that for that process in that situation it wasn't economical. I have seen this case a few times.

Another case I've seen a few times is when there are only a small number of parts on the bottom of the board. Two-sided reflow takes some special handling, and for a very small number of parts it's more cost effective to run the normal process for the top side, then add on the small number of bottom parts by hand.

More stuff is hand soldered than you might imagine in places where the labor rate is low. A factory one of my customers used in China had a whole manual assembly line right next to the more automated pick and place and reflow line.

Again, it is highly unlikely what you heard is due to a part needing to be hand soldered due just to the part itself. A assembler may have said they "need" to hand solder a particular part, but that was due to their process and tradeoffs, not inherent to the part.

Answer 2

I can think of a few realistic situations that could have been distorted just slightly to get someone thinking there's SMT parts that can't take automated soldering:

• An old part (SMT or through-hole) designed for tin-lead solder process might not be able to take the heat of a lead-free solder process.

• Some SMT parts might require reflow soldering rather than wave. Or wave might require an added process step (and added expense) of gluing them down to the board to avoid the wave washing them away.

• Some parts can't be aqueous washed. This can make them incompatible with some solder processes, although "no-clean" is more common nowadays. The only case I've run into was a through-hole transformer, but I could imagine it happening with an SMT transformer, relay, or switch.

Someone who'd had to change a design because of one of these situations might only remember, a few years later, that there are SMT parts that aren't compatible with automated soldering.

Answer 3

At my employer, more TH parts are hand-stuffed than SMT parts. In general, the industry is trying to make PCB assembly more automated, not less automated.

It is common to have over 1,000 parts on a PCB these days, and hand-stuffing those would be very labor intensive, error prone, and expensive. If a PCB with 1,000 parts was stuffed by hand, and it took 5 seconds per component (I'm very optimistic), then that PCB would take 1 hour 23 minutes to stuff. It it took a more reasonable 15 to 60 seconds per component then things look even more unreasonable at 4 to 17 hours.

To make things more interesting, parts like BGA's, QFN's, 0402's, and 0201's are more reliably soldered down with an automated process than when hand soldered.

Answer 4

Heat limitations are solved in several ways. By cold air or covered dry ice beds. Automation should be using a dry refrigerated cold box. Fans circulate the cold air through the box from two ends. One end sends cold air as the other circulates it back to refrigerate and blow again. Then blows direct cold across the circuit boards components. The automation line lays the board made to rest onto the cold air box opening with rubber seal grommets to rest the circuit boards. Soldered behind the board from top. Problem solved. They used this at Texas Instruments in the 1970's through 1980's. It will require a climate controlled automation area though so condensation won't become a problem.