Air Feeding Components: The End for Vibratory Feeders?
By John Pompea, President, Contact Systems, Inc.
reprinted with permission from October 2006 issue of U.S.Tech
Over the last decade reliability of pick and place machines has improved to the point that they are considered a commodity product. Yet an important feeding device, the ubiquitous vibratory feeder has not been improved much since its inception. Vibe feeders are used to feed components supplied in plastic tubes (sometimes called sticks). While tape components dominate the SMT world, contract assemblers are often required to use tube components because the minimum quantity available in tape is typically 250 or more components.
Although the vibratory feeder has become the primary feeding device for tube components, it is inherently a bad concept. Vibration is actually a back and forth motion in one axes which together with a little help from gravity move the components in one direction. (Sort of) The component’s forward motion can be tweaked by changing the amplitude, frequency and duration of the vibration. However even with this tweaking the feeding is unreliable because the weight of the component stack varies with the number of components remaining in the tube. You can tweak the feeder for a full stack and find that as the tube empties the last few components will not move at all or in some cases actually move backwards. During vibration the components are pushed together end-to-end up to a stop rail. When picking the component the friction between it and the front edge of the rail can cause a miss pick.
To prevent the picked component from lifting the one behind it the pick window should be carefully adjusted to the components size. However many vibe feeders have a pick window for the largest part of a group. For instance the SO 14-20 will have a window that allows one and a half S014’s to be exposed. In this case mold flash (which causes the components to stick together) will cause the second component to lift when the first one is picked which can also cause a miss pick.
Tubes are usually mounted in the vibe feeder ski slope fashion with the angle of incline varying from 10-35degrees. This incline helps the last few components in a tube actually feed. However the transition from incline to horizontal will cause components to “shingle” which also causes miss picks. Most vibe feeders (but not all) shut off during the pick cycle to reduce miss picks caused by the moving component.
Then there is the problem of loading and unloading components, which is tedious and prone to spillage. On most of the feeders the tubes are mounted on the vibe head. To reload on the fly the operator has to feed one tube into another. Lining up the ends for a smooth flow is difficult especially for tubes that use tacks as stoppers, When the tack is removed there is often a burr left on the tube which stops the flow.
When we launched our C5 series P/P machine we were determined to take a fresh look at stick feeding. Our goal was to make a feeder, which feeds reliably and is easy to load, unload and replenish components on line. Belt feeding was considered but did not meet the criteria. Belt feeding while more reliable than vibratory feeding has its own problems. To return components to a tube from a belt feeder is tedious manual process. Typically the operator picks them up one at a time with tweezers. Belt feeders are costly to make.
Thus they are usually made wide with a multitude of lanes dedicated to one component type.
We began experimenting in our lab with moving components by a puff of air and discovered it was the perfect solution. The components need to be in an enclosed track, which means their needs to be an easy way to pour them from the tube into the track. For this we designed different tube adapters, which makes loading and unloading simple and foolproof. The air track feeder consists of a base assembly that plugs into the feeder bank, which supplies air and power and a track assembly, which mounts on the base. There are many components supplied in tubes so we designed a multitude of 39mm wide track assemblies, each one dedicated to different component types.
Customers are frequently requesting tracks for new components such as crystals and pushbuttons, which we design, build and add to our inventory. Each track assembly has 1- 4 lanes depending on component size. For instance the PLCC 84 has one lane and the sol track has 4 lanes. Each lane has an adjustable window to accommodate varying component lengths. Each track assembly takes four 8mm tape feeder lanes.
To improve pick reliability the stop rail is backed off the front edge of the component prior to the pick cycle thus eliminating friction. A shutter closes the pick window during the air puff to assure smooth component flow.
Moving components by air is so reliable and simple it is amazing it has not been used before on tube feeders. My prediction is that it will completely replace vibratory and belt feeding within a few years.
