SMT Quick-Tips 6: Selecting a Selective Soldering System
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SMT Quick-Tips 6: Selecting a Selective Soldering System
How Selective programming works
Hybrid boards can be challenging to program because of the interface with SMT components and the nature and arrangement of addressable, through-hole points. Figuring out the best sequence to optimize production speed is usually a combination of common sense along with available software features.
The first step is to “acquire” the board into the software, and there are a number of methods to get started:
- Scan the bottom of the board with a flatbed scanner
- Take a digital photo of the board, such as with a cell phone
- Import data from a CAD file
- Teach the software
Starting with the last one, the Teach-In method is no longer widely used because it can be more difficult to manage than the other methods, which are also far more common.
More often than not, if a board has been designed in a CAD system, using the exported data will result in the easiest and most reliable method of importing the data. This data is known as a Gerber file, a 2D vector file that describes everything about the board for manufacturing, as well as populating and assembling them. It defines multiple layers of images that make up the board, including the size of pads, centroids, through-hole locations, etc.
Once the board has been acquired, the methods for programming the board vary widely, so evaluating the available methods against your particular processing needs will help guide the selection process. It’s easy to add expensive bells and whistles, but for a low volume application or shop, they may not be necessary. On the other hand, some medium to high volume applications will benefit significantly from the time-saving options that improve productivity.
Importing board data
Software for Selective soldering machines is nearly always hosted on an offline PC. The idea is to import the board data, whether it’s a visual representation from a scan or photo, or from a digital Gerber file, then scale its length and width on the computer screen to match the physical dimensions of the board itself. So, now that the board image has been imported into the software, it appears as an overlay of the coordinates of the points behind the board image.
With a simpler software interface, additional PCB data may need to be entered (In addition to the border and size of the board) – such as all the solder process data (temperature, preheat speed, preheat dwell), as well as the up/down Z-axis speed .
And this is where it gets tricky.
Selective solder is not just a pattern of dip points on a board. It also represents opportunities to drag solder across multiple points without changing the elevation, or height (Z), from the board. Furthermore, if nozzle changes will be necessary to accomplish some solder operations, they should be minimized to reduce dead time. On some boards, using a single nozzle to perform less efficient operations than would otherwise make sense for a different nozzle may still be faster than a time consuming nozzle change.
A good software package will be intuitive and easy to use. And while many of these decisions can be made by a thoughtful and experienced board programmer, some sophisticated software packages offer optimization tools to achieve the best pattern for complex board geometries. Such software packages will, along with the judgement of the operator, be capable of deciding whether changing a nozzle would be advantageous or not.
NOTE: If nozzle changes are dictated, they must be done manually. The machine interface will alert the operator any time there’s a manual operation to be performed.
As the operator defines how the board will be processed, the software will prompt the user to instruct the machine what to do at each point or drag selection. Typically, the default command is to flux, preheat and solder each contact at one time, then change the nozzle height and move on to another selection; however, there may be special cases where the board design may require handling some of the operations separately. That flexibility is key if it’s important for your board production.
After the board has been configured, the file is sent to the machine to begin processing. Most software programs have a menu option that allows for easy download of the saved program to the machine controller, usually via a serial, USB or Ethernet connection.
How to evaluate software
The Selective Soldering machine you buy will always offer its own software; however, if you like the machine but its software isn’t exactly what you want, you can often purchase a third-party program which can also export the formatted board back to the machine.
Any respectable machine manufacturer will offer a free demo version of their software for you to evaluate before purchase. It will allow you to see how easy (or difficult) it is to import graphics files, configure them, and program special circumstances that may arise in your production routines.
As mentioned earlier, many suppliers offer lots of time-saving capabilities with their software, but a good rule of thumb is to test drive a number of different types to find your sweet spot, which will be a balance between performance and price. There’s no need to pay more for a capability that you will rarely use. A trial will also quickly help you decide whether the software will be easy for your staff to learn.
Use patience in your evaluation process. Every manufacturer’s software is very different from the other’s, meaning there’s very little uniformity among Selective programming routines compared with what you may experience in other assembly processes. But choosing a Selective machine is a major investment, and it deserves any amount of time you can afford to give it a proper evaluation.
Buy the right machine for the intended purpose
Selective soldering is often touted as being the “latest and greatest” but it’s not the best technology for all boards. The fact is, selective is a much slower process than wave because it has to process each lead or closely spaced set of components one at a time. Selective is the only way to solder through-hole components in a production environment on a mixed technology board or if there are obstructions on the underside of the board. But wave soldering is by far the fastest way to preheat, flux and solder a board with only through-hole components.
Wave soldering dates back to a time even before integrated semiconductors were used, so it’s not a new technology. However, there have been many advances to the equipment as the industry has evolved, and it remains a very effective and cost efficient manufacturing/assembly method to this day. No technology can match it for speed and no other method can improve on its joint strength.
Selective soldering is becoming much more common today, for very good reasons. It can process through-hole components on boards with mixed technologies, multiple shapes, patterns and devices; it is significantly faster and more reliable than hand soldering, so it’s a natural step up from a prototype setting to a production routine; once programmed, recipes can be archived to perform the same operation in exactly the same way in the future, ensuring reliable repeatability.
And this is where Selective Soldering shines.
Understand the process that’s best for your assembly
There are myriads of standard nozzle designs for performing dips and drags, and they are limited only by your imagination or the complexity of the board. Selective gives you many ways to tackle a soldering routine, so you want to think about the sequence, the nozzle configurations you use, and the best way to save processing time. It may make sense to buy a custom nozzle if you have a high volume board with a unique profile. After all, nozzles aren’t very expensive and they last quite a long time.
Think about the future
Perhaps you’re considering the acquisition of a Selective Solder machine for specific new project, or you’re a contract assembly shop expanding your offerings to handle custom configurations. Look beyond the immediate future and think about how your needs may grow. Some Selective manufacturers can offer a base machine that allows for the addition of future modules to expand your capabilities. But some don’t, so be sure to ask.
Additional modules can come in a number of formats, allowing you to double or triple your production throughput. You may be able to get them combined in pairs to perform two operations on the same lead, or get separate modules that only perform a single fluxing, preheat, or solder operation.
Beyond modules, some manufacturers offer automatic conveyors which allow the operator to stage boards and feed them automatically when a previous board is complete. This allows one operator to perform multiple functions rather than keeping his attention on a single machine.
Programming Options
As described earlier in Selective Soldering Part 4, hybrid boards can be challenging to program because of the interface with SMT components and the nature and arrangement of addressable, through-hole points. Figuring out the best sequence to optimize production speed is usually a combination of common sense along with available software features.
The two best ways to “acquire” the board into the system are by scanning the board or importing CAD data from a Gerber file, although in a pinch, you can also take a digital photo.
With more advanced programming options come higher costs, but they often pay off in greater efficiency and throughput.