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April 29, 2013

Component Shifting

We have a problem with chip component shifting that is detected during automatic optical inspection. The shifting is not limited to single component, it changes daily some times at one location, some times on other location.

What is the likely cause? How can we overcome this problem?

V.E.

Experts Comments

Situation: Components moving or shifting from the position at which they were placed.  

Most times this is related to the components floating on the molten solder and the equipment has some vibrations which could impact the locational position of those components. The vibration in a reflow oven may not be felt by the people in the area but could be caused by a truck going by outside the building or a piece of equipment moving around within the building. Although the components are moving from their placed position, there was no mention as to whether they moved to a position where they were not acceptable, such as defined in J-STD-001 for Class 2, and 3 assemblies. If the movement did not cause them to be rejectable then there is nothing to worry about.  

The other issues to consider are the amount of solder paste deposited on the pads and the size of the pads or land areas. If the pads or land areas are too large then again excess solder will be deposited which will allow the component to float and move around, so check these condition. The paste deposition should be around 6 mils or less for small chip component and the pads just slightly larger than the component termination.  

There are lots of unknowns in the question, so I've answered it as generally as can be answer with the information provided.
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Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
One possible cause could be board support related. If the board is not being supported properly the board will experience "bounce" or "vibration" created by the placement force of the machine. This outcome can also be intensified by paste drying out over time and losing its tackiness. Check to see if the board is supported adequately and if not, add support. There are very good board support tools on the market that will reduce this bouncing and improve you placement accuracy such as the Red-E-Set by Production Solutions.
Doug Farlow
President
Production Solutions, Inc.
Mr. Farlow is President at Production Solutions Ins. where they manufacturer the Red-E-Set family of board support systems. His expertise is board support solutions for screen printers, pick and place machines, chipshooters, dispensers and AOI machines.
Reader Comment
It is unclear whether the component shifting that you mention refers to the actual part itself or the inspection boxes (ROI's) that are trained in your Automated Optical Inspection (AOI) system.  If the actual components are shifting that could be due to a number of reasons (i.e. pick and place machine, solder paste deposition, reflow oven settings, etc. 

Inspection boxes (ROI's) that appear to "shift" can occur due to a number of factors. All AOI systems have some sort of alignment feature to align the trained inspection boxes with respect to the actual components placed.  Typically 2 or 3 global fiducials are trained and tested upon for every board that enters the system. However, factors such as board warpage and panelized boards may cause some components to not be precisely aligned with respect to the trained inspection boxes.

Most AOI systems offer board warpage compensation through a conveyor clamping system and/or board support pins. For panelized boards, training block fiducials  for each individual panel should help with individual block alignment. Some PCB layouts include local fiducials near critical components (i.e. IC's and BGA's) in which you can use for additional alignment compensation within a certain area. There are times were components may move around because of a large pad design (i.e. DPAK packages). In this scenario, there are options to train an "anchor" marking that will reposition the inspection boxes with respect to where the component shifts on the pad.  

Kevin Garcia
Regional Sales Manager
Nordson YESTECH
Nordson DAGE
The way this issue is defined is not completely clear. Do you perform an AOI step right after pick and place or after reflow? If you do it before reflow then you can address the issue in accordance with:
  • PCB flatness (bow and twist measurements)
  • stencil design
    - Is there enough volume of solder paste on the pads?
    - Is the paste deposition accurate (x, y positioning) in regards to the pads
  • pick and place
    - proper board support
    - boards level measurement (if the feature is available on your equipment)
    - component package proper definition (width, length and height), centering (optical and mechanical), placement accuracy (x, y positioning) and placement force
    - the type of component(s) that shifts: you might be able to find a similarity on the pick and place tool type used for all these packages, bad tool that will result in vacuum problems, centering issues, pick and place fall-outs
    - conveyors: vibration, belt integrity, fetch and release conveyor alignment, component reel "tails" hanging behind the machine and touching the board (with only paste on it or with paste and components) in the load or release operations
    - operator error at loading/unloading (if the process is manual) or "bumping" the parts before sending them through the reflow equipment
  • the AOI equipment
    - vibrations
    - operator error (if manually load)
Now, if you perform AOI after reflow, add the reflow equipment as a potential to induce failures:
  • equipment leveling
  • vibrations and 'snaps" (on the chains/rails and/or flat belts)
  • blowers problems: issues can occur due to mal-functional blowers especially if they are close or in the liquidus stage of the solder paste
  • boards excessive flexing due to mass, design, thickness, component population, temperature and speed
  • operator error: the components can be accidentally bumped when sending the board through the tunnel
As you might notice already, I always include the operator as a potential source of defects. Usually they are involuntary but a close observation of their activity cannot be neglected in your investigation. Good luck.
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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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