|Ask the Experts|
April 21, 2020
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?
|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.
Vice President, Technical Director
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.
Production Solutions, Inc.
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.
Regional Sales Manager
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:
Engineering and Operations Management
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