Passing the Boards
One of the electronics industry?s dirty little secrets is that the average printed circuit board (PCB) assembly line is only 40 percent efficient. With that kind of efficiency, small changes can make a big difference. At a high-volume shop, a boost in efficiency of just 5 to 10 percentage points can add thousands of dollars to the bottom line.
But if CEOs aren?t exactly thrilled to authorize new equipment purchases to address the problem, there is one low-cost technology that can make a difference: board handling equipment. Conveyors for PCBs aren?t long, but don?t let their size fool you. They are more flexible and interface with more types of machines than conveyors for most mechanical assembly applications. By balancing production, maximizing equipment utilization and increasing floor space, PCB conveyors can help assemblers squeeze a bit more output from their lines.
Some PCB assembly machines can pass boards directly to each other, but many require a conveyor. "A placement machine can usually pass to another placement machine, but you can?t go from a placement machine to an oven," says Pat Ortiz, director of sales at Nutek USA (Longmont, CO). "And if you need to do something to the board, like invert it, you?ll need a conveyor."
Like the machines they serve, electronics assembly conveyors handle PCBs by their edges. Usually, the boards ride on thin, static-safe belts, but conveyors can also be equipped with metal pin chain or plastic chain. Either way, assemblers should provide a clearance of 3 to 5 millimeters from the outer edges of the board, so the conveyor has room to do its job.
The distance between the belt rails can be adjusted from 2 to 18 inches, and most suppliers can custom-build conveyors for larger boards. The width can be adjusted with a hand crank or a motor. Some conveyors automatically adjust rail width. If the rail width changes on the machine ahead of the conveyor, sensors detect the change and automatically adjust the conveyor?s rails to match.
"If you?re changing conveyor width once a day or less, a hand crank is fine," says Tom DiNardo, president of Simplimatic Automation (Lynchburg, VA). "If you have to change widths more often, you may want to consider programmable adjustment."
As for board length, most conveyors can handle boards ranging from 4 to 20 inches long.
Board thickness is rarely an issue. However, when handling thin boards, it?s good if the top of the belt rides above the bottom of the rail guide. "In many conveyors, the belt and the rail guide are at the same level, so a very thin board can be caught between them," says Rick C. Carr, general manager of Kanetic Inc. (Lake Barrington, IL). "Our equipment has successfully transported boards as thin as 0.004 inch, which is as thick as a piece of paper."
PCB conveyors are rarely long. Most are 20 to 40 inches long. "A nice feature of our conveyors is that their length can be adjusted ?3 inches," says Carr. "That small bit of adjustability can make a big difference when laying out a line. Instead of moving a big machine like a chip shooter or an oven, you just shorten or extend the conveyor."
Transfer speeds range from less than 1 ips to 20 ips. Ideally, the conveyor should match the speed of the incoming and outgoing machines. "You don?t want abrupt starts and stops, because surface mount parts on wet solder paste can skid around a bit if they?re handled too sharply," says DiNardo.
Presence sensors on either end of the conveyor detect PCBs so they can be stopped for inspection or queued. The sensors also signal the downstream machine that a board is on its way. "Fixed field sensors allow you to run boards of varying color interchangeably through your line," says Carr. "No sensitivity adjustments are required."
ChoicesBesides the simple, short unit for transferring boards from one machine to another, electronics assembly conveyors are available in various configurations to perform specialized tasks.
Telescopic conveyors create a passage into the PCB assembly line to give the operator access to the rear of the production equipment. The conveyor slides across the opening when a board is requested, but returns when the board is released, leaving an open passage in the line.
A gate conveyor performs the same function. But instead of retracting, this conveyor has a section that tilts up and down, like a drawbridge.
"As PCB assembly lines get longer, access to the back of the machines becomes an issue," says DiNardo.
Several options are available for changing the direction of the line, or for transferring boards to adjacent lines. L-type conveyors rotate boards 90 degrees so assembly lines can turn a corner. T-type conveyors transform from one line into two lines or vice versa. X-type conveyors turn one line into three lines, three lines into one line, or two lines into two lines.
Lifters are for lines with more than one level. They can be used to separate good and bad boards and for increasing capacity of certain process equipment. The board is lifted or lowered together with the conveyor.
Inverters are used to flip double-sided PCBs. Two belts hold the board while it?s gently turned upside down.
Some conveyors are incorporated into workstations for manual assembly operations. Often used near the end of the line for odd-form assembly, these conveyors typically have scalloped tabletops, footrests and connections for static-control devices. Parts bins, task lighting and other equipment can be attached to uprights at the rear of these stations.
Other conveyors have a lifting option. Metal fingers lift the PCB from the conveyor, enabling the following PCBs to continue downstream. This feature can be used to cull rejects from the line or to enable an operator to visually inspect a board without interrupting flow.
Automatic loaders and unloaders are placed at the beginning and end of the line. "The industry commonly refers to these devices as board breakers, because many of them will keep pushing a board into or out of a magazine even if there?s an obstruction in the way," Carr says.
To avoid that problem, assemblers should look for loaders and unloaders that have force sensors to detect pressure on the board and stop the insertion process until the problem is corrected.
A cousin of the loader and unloader is the buffer. There are two types of buffers: first-in, first-out (FIFO) and last-in, first-out (LIFO). "Buffers are used when one machine processes boards faster than the next machine can handle," Ortiz explains. "For example, buffers are often placed between placement machines and reflow ovens, because ovens are much slower."
Buffers can also be placed after test and inspection equipment, to separate good boards from rejects.
Suppliers ListBliss Industries
Conveyor Technologies Inc.
Arlington Heights, IL
Integrated Production & Test Engineering
Lake Barrington, IL
Huntingdon Valley, PA
Nix of America
San Jose, CA
Fairless Hills, PA
Quickdraw Conveyor Systems
Seika Machinery Inc.
Universal Instruments Corp.