SMT Placement

Surface mount components are placed on a printed circuit board after deposition of adhesive or solder paste. Placement equipment is commonly referred to as Pick And Place Equipment. The components can be placed on the board by

1. Manual placement
2. Automatic placement

MANUAL PLACEMENT

Manual placement of surface mount components is not reliable and can be used only for prototyping applications. The following are some of the problems associated with manual placement of components.

1. Most passive components do not have any part markings, and therefore, the possibility of part mix-up is very high.
2. Placement of components in wrong orientations due to operator related errors
3. Placement accuracy will again depends on the operator and it is very difficult to obtain good placement accuracy with manual placement.

In component placement, there are two main functions, pickup and placement. In manual placement, the components parts are picked up either by tweezers or by a vacuum pipette. For passive components tweezers are adequate, but for multi-leaded active devices, a vacuum pipette is very helpful in dealing with component rotation and is recommended when working with manual placement of components.

The following are the benefits of using manual placement machines

1. Very low capital cost
2. Ideal for prototype development due to low turnaround time and low cost
3. For low volume production manual pick and place machines with vacuum pickup and placement are preferred.

AUTOMATIC PLACEMENT

Automatic placement machines are required for high volume production as the manual method turnsout to be very slow. It is also possible to achieve consistent quality levels with automatic placement equipment. Vision capability is required for placement of fine-pitch components and it may add to the overall cost of the equipment. The following are the important parameters while selecting an automatic placement machine.

1. Placement speed

2. Requirement of placement of fine pitch or very fine pitch components

3. The variety of components that the machine should handle

5. Other features like CAD data downloading for reduction in programming time

6. Types of Feeder that the machine should support

7. Ability to dispense adhesive

8. Need for component testing

The user has to choose the placement machine appropriately depending on the application on hand and the future needs.

Surface-mount components are placed along the front (and often back) faces of the machine for the Inline Machines and for the Offline Machines the components can be loaded on all the four sides depending on the requirement. Most components, large and small are supplied on paper or plastic tape, the tape reels are loaded onto 'feeders', mounted to the machine. Larger ICs are sometimes supplied arranged in trays, which are stacked in a compartment on the machine for this purpose.

Through the middle of the machine there is a conveyor belt, along which unpopulated PCBs travel, and a PCB clamp in the centre of the machine. The PCB is clamped, and the nozzles pick up individual components from the feeders/trays, rotate them to the correct orientation and then place them on the appropriate pads on the PCB with a great deal of precision. Most Placement machines now can optically inspect components on picking, to ensure that the correct component has been picked, that it has been picked successfully and that it is in the correct rotational orientation. The components may be temporarily adhered to the PCB using the wet solder paste itself, or by using small blobs of a separate adhesive, applied by a glue dispensing machine.