Before discussing the benefits of fixturing in Surface Mount Technology (SMT) it is worthwhile to clearly define some terms that relate to the concept of using fixtures in a manufacturing process. Fixturing and its similar concepts were originally used in woodworking. It then become commonplace in the world of engineering and manufacturing as a way to produce the exact same product multiple times.
Fixtures, jigs and templates have been used in woodworking and manufacturing for many years. The terms are often confused and used interchangeably. Let's use the very simple example of replicating a house key key to explain the differences and then zero in on how each of them are used in surface mount assembly.
A Jig is a tool custom designed for the express purpose of directing or controlling the location or motion of another tool. In our example, we clamp a master key into one side of a machine that will then trace the master and produce an exact replicate. Machine tool manufacturers have inventories of jigs they use when they need to create exact replicates of a product. Likewise, EMS companies now have libraries of PCB "jigs" they can reuse as required.
The Template in our rudimentary example, is the original key. A template may be the actual sample being replicated as in the case of our key. They may be also a model of something we want to create or a drawing that can be cut out like when installing a door knob. Templates can take many forms. In the case of a Printed Circuit Board, a template or schematic may be placed on a rectangular board and then cut into the desired shape using a template. A typical PCB manufacturer also has libraries full of "templates" they refer to as footprints or schematics.
The Fixture in our example would be the clamping device that holds the key in exactly the right position every time without fail. Fixtures also can take many different shapes and forms.
This brings us to fixturing in the world of Surface Mount Technology. Most mid volume EMS companies today are use fixtures that hold multiple boards in place.
5 Benefits of Fixturing in Surface Mount Technology:
1. Flexibility - Boards can be mounted in many different ways
2. Ease of Replication - Once mounted, multiple iterations and processes may be performed
3. Quality Control - Because the processes are done exactly the same each time, the quality can be dramatically improved
4. Reduced Waste - Once the process is defined, it works the same each time so there is minimal waste
5. Reduced Cost - All of the above result in lower costs to the electronics manufacturer and these cost savings can be passed on to the customers.
Why aren't all boards fixtured? Because fixturing requires additional labor, time, and capital investment. Each circuit board must be hand place and taped securely in the fixture. Assembly is complete when the fixture leaves the reflow oven, and the fixture is very hot and must be allowed to cool. If the takt time of the line is fast 1-3 minutes, a lot of fixtures are required to allow for this cooling. And automated board loading/unloading is not possible with fixtures.
