Like it or not, many printed circuit boards have odd-form components: axials, radials, header strips, DIPs, SIPs, connectors, transformers, telephone jacks, relays, TO-220s, displays, LEDs, switches, pin grid arrays—even small daughter boards.
Often, these components are inserted, and even soldered, manually. However, advances in robots, grippers and machine vision have made automation of odd-form assembly (OFA) justifiable at the right production volume.
The primary reasons to automate OFA are quality improvement and rework savings, says Tawnya Henderson, precision solutions marketing manager for Adept Technology Inc. (Livermore, CA). "If a facility is running two shifts, 5 days a week, it’s a good candidate for automation," she says. "A return on investment of about 12 months can be realized based on rework savings alone.
"At lower production volumes, the justification can be more difficult, unless the cost to scrap or rework a product is significant enough to justify the equipment investment based on quality alone."
How the board will be soldered also influences the decision to automate OFA, says David Vanaken, sales manager at Integrated Production & Test Engineering (Alpharetta, GA). If the boards will be sent through a solder wave, automating OFA may be less justifiable, because assemblers have little difficulty inserting the components. However, if the pin-in-paste method is used, automation is easily justified. "It’s very difficult for operators to insert components into holes with paste inside," he says. "If they move the component too much while it’s in the paste, it will cause a bad solder connection. So if you’re doing pin-in-paste, you should really use a machine for odd-form assembly, because it will be more accurate."
Feeding Odd-Forms
Odd-forms can be presented to automatic insertion equipment in many ways, ranging from tape-and-reel systems to vibratory bowls. Which method to use depends mostly on the component, though the machine, board mix and production volume also influence the decision. In fact, it’s not unusual for one machine to use more than one feeding method.Radials and axials are fed with bandoliers, while pins, tabs, terminals and sockets are usually fed on continuous strips.
Headers, connectors and other components can be fed with a tape-and-reel system, as long as they are small and relatively short. The components can ride directly in the plastic tape, or they can ride in recessed pockets covered by a plastic film.
For larger components, such as D-Sub connectors, telephone jacks, relays and transformers, a tube feeder is a good alternative. In this method, components are packaged inside custom-made plastic tubes. Components can be stacked in the tubes vertically, one atop the other, or horizontally, end to end.
Some components are so oddly shaped that the only way to feed them is with custom-made, vacuum-formed plastic trays. "If you have very large components, you might only get 10 or 12 components in a tray, and the tray itself could be 1 to 2 inches tall," says Vanaken. "That creates storage problems. You can’t get many trays on the machine, but at least you can reuse and recycle them."
Robodyne Corp. (Minneapolis) has developed a hybrid between trays and tape-and-reel systems. Called Softray, the product consists of a thermoformed tray with a protective lid, says Joseph Alvite, Robodyne’s president. Indentations in the lid fit into indentations in the tray like a zipper. Inside the feeder, these indentations serve the same purpose as sprocket holes in carrier tape.
Vibratory bowls are the most expensive and least flexible option. They should be reserved only for the highest volume applications. Lead tangling and electrostatic discharge are concerns when feeding components with vibratory bowls.
"Custom feeding methods are used when it makes sense," says Kelly O’Brien, product marketing specialist for odd-form and final assembly equipment at Universal Instruments Corp. (Binghampton, NY). "The cost and volume of the components are considerations. Is it more advantageous to incur the incremental cost of packaging the component? Or is there a reasonable return on investment for a custom feeder?"
Gripping and Inserting
Surface mount components are easily handled with a vacuum nozzle. Odd-forms, on the other hand, usually require a gripper. To complicate matters, a gripper that handles one component may be unable to handle a different one.Equipment suppliers have come up with two ways around the problem. One solution is to mount several different grippers on a rotating insertion head. Another solution is a servo gripper that can handle more than one component type or size. For example, several manufacturers offer grippers that can pick up components with either a vacuum or a pair of fingers. Both grip pressure and finger movement are programmable.
One detail board designers often overlook is clearance for the gripper, says Vanaken. To ensure that the gripper has enough room to fully seat the component on the board, designers should provide at least 0.16 inch of space around the insertion site.
It’s one thing to place a surface mount component on a pad; it’s another to align dozens of tiny pins with tiny holes and insert a component with just the right amount of force. For problem-free insertion, it’s a good idea to use machine vision to check a component’s leads prior to insertion, says O’Brien. This will ensure that a component with bent leads is not jammed into the board.
Similarly, sensors on the insertion head can detect force fluctuations that could indicate a misaligned component or misdrilled holes. "When the hole-to-lead clearance is tight, the ability to tactilely sense misinsertion without bending leads becomes very important," says Henderson.
IPTE’s Vanaken warns assemblers not to rely on fiducials for aligning through-hole components prior to insertion. "Fiducials don’t help you a whole lot," he explains. "The fiducials are made when the image of the circuit is created on the board, so they’re a good guide for placing surface mount components. But odd-forms are placed in holes, and those holes are made at a different time than when the circuit is traced on the board. So, there might be a tolerance between the two. For inserting odd-forms, we recommend aligning the board with tooling pins."
Board support during insertion is advisable if the component has a retention feature and will require a little extra force to insert, says O’Brien. It’s also worthwhile if the board could warp or sag.
Clinching is not required if odd-forms will be soldered with the pin-in-paste process. However, for quality reasons, it’s wise to do so if the board will be wave-soldered, says Henderson. Lead clinching keeps parts from falling over or becoming unseated during soldering. The clinching mechanism also supports the board while components are inserted.
There are two methods of clinching. One approach is to use two dedicated clinching mechanisms. This approach is limited to clinching leads in the inward and outward directions and to rotating in set increments. Another approach is to use a single, programmable clinching pin. The advantage of this approach is that leads can be clinched at any angle and in any direction.
"An important feature of clinch mechanisms...is their ability to clinch very heavy leads," adds Henderson. "Heavy leads of up to 0.06 inch are typical in odd-form assembly, especially in automotive applications."
Soldering Odd-Forms
Odd-forms can be soldered with a solder wave, a solder fountain, a selective soldering robot or a reflow oven. Again, the best method to use depends on the components and the number of odd-forms on the board."If you just have one or two odd-form components, reflow is the best solution, because it will minimize your investment," says Vanaken. "If you have a lot of odd-form components, then of course you’re going to use a wave. If you have a component that can’t take the temperature of a wave or reflow, then selective soldering is the best solution."
