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Selective soldering equipment was originally developed to do what neither hand nor wave soldering could do — automate the soldering process for individual through hole (T/H) components or groups of components without disturbing nearby SMT components or groups of components. Thus, selective soldering emerged to fill this need, and the technology continues to develop with the advice and valuable input from users engaged in day-to-day production soldering operations. Alan Cable, ACE Production Technologies Inc., shares some of these technician's tips, and how they improved selective soldering.
It’s no surprise that electronics manufacturing is always seeking faster cycle times; higher throughput volumes per shift directly impact the profitability of an operation. In particular, selective soldering system users wanted to find ways to speed cycle time. By studying the various motions of the equipment in operation, selective soldering equipment engineers found a way to create a shorter soldering cycle by trimming unnecessary motion where it would make the greatest impact, without sacrificing quality or precision.
The selective soldering systems in this R&D project* utilize a programmable traveling flux applicator and a mini solder wave; this wave is mounted above a 30-lb. solder pot, which carries a soldering nozzle and flux head that moves in the X, Y, and Z axes. The application of the flux is rapid and precise with no lost motion. For this study, the focus is in the relatively slow soldering portion of the process.
A typical soldering program consists of commands that direct the solder delivery system to apply solder to specific X/Y locations at pre-programmed speeds. The amount of thermal energy required to solder specific component locations is dictated by the thermal mass of the electronic components and the PCB substrate. X/Y travel speed, solder pump speed, and solder dwell time (the amount of time that solder flows to a particular location) are fundamental parts of the soldering cycle time equation and not subject to expediting.
Z-axis Travel — The Necessary Annoyance
For every X/Y program segment, there is also a Z travel segment that directs the solder delivery system to rise to the soldering position, and then subsequently to lower to a travel position. However, the Z travel stroke does not contribute to the process, and in fact negatively impacts the cycle time. Since the Z axis is not a productive component in the cycle, expediting that travel will not affect soldering parameters, but it may shorten the overall cycle time.
By studying the hundreds of soldering programs generated by our process development group, our engineers discovered that 20 to 30% of any given soldering sample cycle time is wholly given to moving the solder pot up and down. If the Z-motion could be sped up, cycle time would improve. The results are summarized in Figure 1.
Figure 1. On the sample board, there are 34 sites to solder that require a Z motion to move the solder wave to and away from the solder site. In this analysis, a travel distance of 0.5" is used for each direction to avoid certain SMT and T/H components. The new machine travels at 5"/sec., up to 10x faster than the current machine. The total time saved is subtracted directly from the process time of the standard machine.
Lighter motion components, a new drive system, and motors that allow precise control of Z acceleration and deceleration profiles were designed into a new Z-motion system, which significantly cut time from the cycle and resulted in a high-speed selective soldering machine. The improvements translate into a 22% increase in productivity, as shown in the graph in Figure 2.
Figure 2. Solder Pot Change-outs Simplified
As an added benefit, the improvements to the Z assembly inspired a better method of removing and exchanging solder pots, which users have also requested.
Selective soldering equipment users wanted a way to facilitate solder pot change-outs. Many contract manufacturers (CMs) are using multiple solder alloys to meet a variety of customer specifications. Thus, the faster those change-outs can be accomplished, the faster the selective soldering machine can be back in production. Even though this solder pot is comparably light (30 lbs.), it is still necessary to maintain good safety and stability as well as prepare the exchange solder pot for use as soon as possible. A service cart was designed that also pre-heats the solder to a nearly liquid state prior to swapping into the machine. Use of the stable cart makes solder pot exchange simple, fast, safe, and achievable with minimal effort. Since the new pot is already pre-heated, it can be molten and ready to run in a matter of minutes once set in place. In Figure 3, the warmer-equipped service cart is shown in position in front of a selective soldering machine, ready to exchange its solder pot.
Figure 3. Conclusion
Listening to customer suggestions always pays dividends to both the equipment supplier and the customer, with resultant better equipment, higher productivity, and better profitability for all.
*ACE developed the high-speed system from its KISS selective soldering equipment.
Alan Cable, president, ACE Production Technologies Inc., may be contacted at 3010 N. First St, Spokane Valley, Wash. 99216; (509) 924-4898; Cell: (509) 993-6571; email@example.com; www.ace-protech.com. Read his other recent SMT article, Selective Soldering: Cost-effective Replacement for Production Hand Soldering