COMPONENT POSITIONING SYSTEMS

 

After each component is picked up and centered in the tool by one of the methods described in the previous chapter, it must then be positioned accurately on the board, in an X-Y position. There are three methods commonly used for positioning:

• Positioning with no feedback system (open loop system)
• Positioning with rotary encoders (closed loop system)
• Positioning with linear encoders (closed loop system)

Method 1: No positioning feedback loop In this system, the motor drives the part to a location on the board defined in the program by the number of steps in each X-Y axis, but there’s no way to tell if it actually ends up in the right place. These systems use stepper motors for positioning.

• a. Pros: Low cost
• b. Cons: Unreliable accuracy; not recommended for high quality production

Method 2: Positioning with rotary encoder In this method, an encoder is mounted directly on the motor shaft and delivers position feedback to the control system; however, it only reports the motor position, and not the actual position of the x-y axis. This is dependent upon the remainder of the mechanical components that make up the machine. These machines can use stepper or servo motors. (and usually associated with cost)

• c. Pros: Low cost; this system is widely used on entry-level machines
• d. Cons: Typical positioning accuracy of +/- .005”

Method 3: Positioning with linear encoder In this method, linear scales are mounted to the machine’s X-Y axes table and an encoder is mounted on the traveling beam that will be carrying the components. This method will report its actual position back to the control system and make corrections to the position programmed, if needed, to within a few microns of actual X & Y location for the component placement (which is typically 12,800 increments – or steps – for each inch of travel). The best machines in the this category uses servo motors.

• e. Pros: Very high accuracy, to within +/- .0005”; very repeatable
• f. Cons: More costly, but necessary for high value production

NOTE: The quality of the encoder (the position feedback sensor), is an important element in the whole system and does affect accuracy.

 

MACHINE CONSTRUCTION

When selecting a pick-and-place machine, you should be aware that its construction will dictate its effective CPH range and footprint, including considerations for the number of component feeders it can accommodate.

 

1. All-welded steel: The most accurate machine will have a frame that is constructed of solid welded structural steel tube. This provides significant stability necessary for accurate positioning and high-speed movement of X & Y axes. This construction method is recommended for ANY production environments, and it will remain stable without requiring ongoing calibration.

 

2. Bolt-together frame: Extruded aluminum or formed sheet metal frame will come with a lower initial accuracy than a welded frame and will need to run more slowly because it can’t handle the rapid inertia shifts of X – Y axis movement. Further, it will likely go out of calibration frequently, which will adversely impact labor time, downtime and yield. (Lower cost usually reflects a weaker construction.)

 

SOLDER PASTE / FLUID DISPENSING

 

Any pick and place machine should be capable of offering fluid dispensing systems. Most common liquids include solder pastes, adhesives, lubricants, epoxies, fluxes, glue, sealants, and more. This is a valuable option when building prototypes or one-off PCB assemblies that do not warrant the cost of a dedicated printer stencil or foil.

 

COMPONENT FEEDERS

 

If the machine’s production will be dedicated to a small number of components and type of job, it’s very easy to identify the number and type of feeders. However, that is not usually the case with contract assembly shops, since they don’t know what type of board and how many different components the next job will require. Some OEMs also need flexibility for a wide range of board configurations, especially if they intend to use the same machine for prototypes and several different production boards. So, it’s useful in those cases to consider a machine with the greatest number of feeder position and options that can accommodate the footprint your space can handle.

 

Types of feeders include:

1. Cut strip holders are usually associated with the low volume world.
2. Matrix tray holders are used for components that are not available on tape.
3. Tube feeders dispense components supplied in tubes.
4. Electric tape (and reel) feeders are usually more costly initially, but offer the best long-term investment. Electric tape feeders are available as single units in a variety of sizes, and cover the range of 0201 components up to 56 mm large components. Many manufacturers now offer a multiple feeder (known as bank feeder). These are available for 8 mm tape, and can come with up to twelve 8mm feeder lanes per unit.

 

Since components are packaged in many forms, e.g., discrete components on tape, quad packs, matrix trays, tubes, cut strips, etc., your choice of feeders would depend on your production but also on any size restrictions you may have. A good starting point is to purchase the most feeders you can get in the footprint you have available.

 

SOFTWARE

 

When considering the purchase of a Pick and Place machine, one of the most important considerations is the software interface. There are three primary goals of a good operating system for users in the low to mid-volume range, defined as up to 8,000 CPH:

1. Maximizing ease of use
2. Providing broad flexibility
3. Optimizing performance