SMT Feeder Setup And Verification

Most wrong-part defects are not caused by a machine placing the wrong reel. They are caused by a reel of the right part being loaded onto the wrong feeder position, or by a feeder that was loaded correctly but picked up the first part from the wrong pocket. Feeder setup is a manual operation performed by a person under time pressure, and it is the point at which the whole placement program can be defeated before the machine has placed anything.

This article describes how a feeder setup is organised, what the failure modes are, how the setup is verified before the run, and how it is maintained.

Why The Setup Is The Risk

The placement program is a file and it does not change between runs. The feeders are physical objects that are loaded, unloaded, spliced, moved and reused, and every one of those operations is an opportunity for an error. The same feeder position may carry a different part in the next run, and the operator has to change the reel, the program and the machine’s own record of what is loaded.

The consequence of an error is not always a wrong part. A reel that is loaded in the wrong orientation feeds the parts upside down, a reel that is spliced incorrectly produces a missing part at the splice, and a feeder whose tape width is wrong produces a part that is picked up at an angle and placed badly. Each of those produces a different defect that points at the machine rather than at the setup.

Loading, Splicing And Reels

The tape is threaded through the feeder along a defined path, and the cover tape is peeled back as the parts advance. A tape that is threaded incorrectly damages the pockets, and a cover tape that is peeled at the wrong tension produces parts that are lifted out of the pocket or that stay under the tape. Both cause a missing part, and both are visible in the feeder before the machine starts.

Splicing is the operation that joins a new reel to the end of the old one so that the machine can continue without stopping. It is done with a splice tape and a defined overlap, and the parts at the splice are lost. A splice that is too short produces a jam, and one that is not detected means the machine continues with an empty tape until the parts run out at the wrong moment. The splice point is recorded so that the parts it consumed can be accounted for.

Feeder Types And Failure Modes

A tape feeder advances the tape by a pitch with a sprocket or a lever, and its failure modes are a worn sprocket that causes a partial advance, a spring that has lost tension and a tape guide that is clogged with adhesive from the cover tape. A stick feeder relies on gravity and vibrates the parts into position, and it fails when the tube is at the wrong angle or when the parts are not uniform.

A tray feeder presents parts in a matrix and needs a program that describes the tray layout, and its failure mode is a tray that is loaded with a different orientation from the one the program expects. A bulk feeder is used for small parts in high volume and is the most sensitive of all to the condition of the parts and the nozzle. Each type has its own check list, and the check is performed before the run rather than after a defect appears.

Verification Before The Run

The verification has three parts. The first is a physical check that the reel in each position is the part the program expects, which is done against the setup sheet by reading the reel label rather than by trusting the position. The second is a verification by the machine itself, where an operator loads a sample of each part into a verification station and the machine confirms the value, the tolerance and sometimes the package. The third is a placement check on the first board, which is part of the first article.

The verification station is the strongest of the three, because it measures the part rather than reading a label. It costs time at setup and it removes the class of defect that is hardest to find later. Where such a station is not available, the physical check is performed by two people for the parts that are critical, which is a cheap substitute for the equipment. The placement rules that the setup serves are described under placement order and pad positioning.

Traceability And Changeover

Every reel has a part number, a manufacturer and a date code, and the machine should record which reel was in which position for each lot. That record is what allows a defect found in the field to be traced to a reel, and it is what makes a supplier complaint supportable. It also allows a partial reel to be reused on a later lot with confidence, because the history is known.

Changeover is the operation that produces most of the errors. A setup that is changed between two products with similar names, or with a shared part, is the classic source of a mixed up reel. The counter measures are a physical separation of the reels for the two products, a setup sheet that lists the positions in the order they are to be changed, and a verification afterwards rather than before. The assembly sequence is described under PCBA development process.

Maintaining The Feeders

A feeder is a precision mechanism and it wears. The pitch that it advances changes with the wear of the sprocket, the tension of the spring falls over time, and the tape guide collects adhesive and dust. A maintenance schedule that inspects the advance pitch, replaces worn parts and cleans the tape path is what keeps the placement accuracy stable over a long run.

The wear shows up first as an intermittent picking error, which the machine reports as a vision failure or a nozzle error. Keeping those reports and looking at them by feeder position identifies the feeder that is degrading before it produces a defect that reaches the board. The rules for the component clearance that the placement needs are described under manufacturable design guidelines.

Nozzle Choice And The Feeder

The nozzle and the feeder are a pair, and a change of one usually requires a change of the other. A part picked from a pocket needs a nozzle that is smaller than the pocket and large enough to hold the body, and the nozzle has to descend far enough to reach the part without touching the tape. A nozzle that is too large picks up the tape as well as the part, and one that is too small leaves the part in the pocket when the tape is advanced.

The placement speed interacts with both. A high pick and place speed gives the nozzle less time to form a vacuum and the feeder less time to settle after advancing, so a combination that works at a moderate speed produces intermittent picks at full speed. When a new part is introduced, the speed is set from the trial rather than from the machine maximum, and the setting is recorded so that it survives the next program change.

FAQ

How often should the setup be verified? Before every production run and after every change of feeder, reel or program. A run that continues across a shift change should be re-verified at the change if any feeder was touched.

Can a wrong part be detected by inspection? Only if the package or the marking is different and the inspector has the information. A part with the same package and a different value is invisible to the eye and is detected by the verification station or by the electrical test.

Is a partial reel safe to reuse? It is, provided the part number and the date code are recorded and the tape has not been damaged. A partial reel with a broken tape or a missing leader should be rebuilt rather than loaded.