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.

Expert Panel Responses

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.

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 your 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
NordsonYESTECH
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 thepick 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 malfunctional 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.

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.

I doubt very much that your issue is due to vibration, based on your description of the problem. I also doubt very much that it is operator-related. These certainly ARE a possibility but things like reel tapes brushing across the surface, operator mishandling, severe bumps, etc. cause major issues, not the slight misalignment only on certain portions of the assembly that you describe. I also doubt vibration from trucks out on the street, or from conveyor vibration, or too much placement pressure, etc., are the root cause because these seldom select only portions of the assembly to disturb; if those issues are present then ALL of the parts on the CCA will display slight misalignment, not just a few here and there.

One thing I have learned is that when using unleaded SAC solder alloys, the wetting tension is greatly reduced. This, along with aged or slightly oxidized components and/or PWBs can cause a lack of wetting tension that helps pull the components into alignment, even in spite of all of the other causes mentioned except, of course, gross mishandling or other more catastrophic causes. I once worked in a factory where the train tracks were less than 30 feet or so from the factory, and ran parallel to the factory for over 900 feet. Believe me, we could feel the vibrations when the train went by! But we never had any issues with misalignment not easily assigned to one of the other causes mentioned by the experts and this was a very high volume shop running three shifts.

Again, while one cannot preclude all of those causes mentioned, I would start with documenting each defect or set of defects on your defect reports and accumulate as much data as possible, but the first place I would look is the date codes of the parts as well as the solderability of the PWB itself. Improper plating frequently does cause issues as you have described over only portions of a given board in a lot, or just a few, and can affect selective portions from board to board.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Submit A Comment

Comments are reviewed prior to posting. You must include your full name to have your comments posted. We will not post your email address.

Free Newsletter Subscription
Circuitnet is built for professionals who bear the responsibility of looking ahead, imagining the future, and preparing for it.

Insert Your Email Address