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June 1, 2016

Question About PPM Defect Rate for Reversed Packages

Which process/equipment has the higher ppm defect rate for reversed packages: SMT pick-and-place or component taping-and-reeling? I have seen some PCBs with reversed packages. The component supplier's tape-and-reel system uses a camera to confirm package orientation before tape-and-reel. The pick-and-place system doesn't have optical pre-placement verification that I know of.


Experts Comments

Very seldom you'll find a reversed package from a component supplier. On the other hand, reversed packages from SMT placement equipment are a little more frequent. This is not because the pick and place equipment does not have optical verification like it seems to be your case but because all SMT components are dependent on the information entered in their part descriptors or product libraries. Many times a change done to one part descriptor could affect one or all components that share the same characteristics on the same assembly. The same change could also affect more than one assembly, in some cases dozens.
Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

We'll have to assume that we are ruling out "systematic" errors, e.g. an entire reel of parts loaded opposite of what is expected or a program error that causes a specific reference designator to be rotated; in other words, what we are talking about is uncommon, sporadic occurrences. If so, then I would suggest that the SMT equipment is highly unlikely to place a part in the wrong orientation. It knows what the orientation is when it picks the part, and what the rotation needs to be on the CCA. If the parts are in fact all correct in the reel, then the part would need to slip/rotate relative to the pick head between the pick point and the camera. Not impossible, just very unlikely.

Some things to do:

  • Look at whether the rates of reversed parts are statistically different between similar packages. If so, then it points to tape & reel issue
  • Pull stock on parts that have had observed instances of reversed parts, put them on a parts counter and run through the entire reel, looking at them under magnification. Reversed parts should stand out. You may need to optimize lighting to see through the cover tape.

Another potential place to look is rework. When we manually replace parts, the opportunity for error is much higher.

Fritz Byle
Process Engineer
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

This is not likely come from the taping and reeling service supplier.

Also the pick and place equipment is going to execute what we are programming it to do. What I have seen in the past:

  • Component "package" description that would not take in consideration the orientation of the part: i.e. a SOT-23 that is defined as non-polarized/no specific orientation will be placed by the machine in any position (a 0 degree or 180 degree) since the machine does not know that there is a difference. All of the components associated with this package will have the same problem
  • Board vibration on the pick and place process - it is not unusual to have parts moving or "flipping" while pick and place process as the speed of most of the machines is impressive these days - keeping the board flat at all times is very important
  • Placement order - placing small parts by bigger parts in the wrong order/sequence can cause the tools' nozzles and the parts themselves to hit the parts already placed in the adjacent areas
  • "hand-place"/rework before reflow is also a variable in your process - the operator can try to fix something that is found on the board and cause bigger issues.
Georgian Simion
Engineering and Operations Management
Independent Consultant
Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.
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