vision placement [vision systems]
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THE GROWING TREND FOR MINIATURISATION HAS CHALLENGED THE ABILITIES OF
COMPONENTS WITHOUT DAMAGE. SOMEWHAT SURPRISINGLY IT IS CONVENTIONAL VISION SYSTEMS TECHNOLOGY THAT HAS EMERGED AS THE ANSWER TO THESE ANXIETIES.
ELECTRONICS MANUFACTURERS TO ACCURATELY PLACE SENSITIVE, HIGH-PERFORMANCE
By Paul Rooimans CJ
ith the trend for products that are 'small and beautjful' gathering pace, the problem of accurately positioning increasingly high performance components is one that urgently needs to be resolved. Sophisticated
components with relatively low yields, such as those used by the micm electmmechanical systems W M S ) and electmnic fabrication industies, are just too expensive to manufactun to lose to poor assembly
Manufacturing industry has developed bigh performance vision systems to ensure right-first-time placement of traditional expensive components such as those made frnm composite metals or machined on multi-axis machinery. However, this technology has, so far, not transferred to the electronics manufacturing industry other than to perform simple inspections of printed circuit boards (PCBs). Conventional wisdom suggests that vision systems cannot be used for placing new-generation electronic components such as ceramic column grid arrays (CCGAs) -they cannot see the connectors on the underside of the component, so how could they?
With manufacturing problems fast becoming the major s o m of chip and board failure as system design technique improve, placement errors of these increasingly complex components are threatening to neutralise their benefits, at great expense. Accurate placement, without causing
component damage, is an issue that needs to be addressed. Vision systems are currently the most reliable form of placement verification available, so can they be developed to see the connectors on the underside of a component?
INCREASED COMPLEXITY An increasingly common type of complex component is the CCGA. These are particularly suited to high-reliability applications requiring hermetic packages and very bigh- density interconnections and higher board-level reliabilitx such as for the space, military and telecommunications industries. CCGA packages use high-temperature solder columns instead of balls to create a higher standoff for more flexible interconnection, and to significantly increase the thermal fatigue life of the package's solder joints.
Today, CCGA components represent a common pmgramming challenge for machine operators, as do highly complex, asymmetric ball grid arrays (BGAs), chip scale packages (CSPs) with very fine pitch, large multi chip module (MCMs), large quad flat packs (QFPs) and tlip chips.
These components are not only complex and expensive but, in many cases, also extremely fragile and easily damaged if not handled properly Whereas QF'Ps only have leads on the sides, BGAs, and CCGAs have interconnections in their arrays, thereby using less space on the board. The demand for these complex components will therefore continue to increase, since applications are getting +
10 IEE Manufacturing Engineer , OnoberlNavember 2004
smaller and, as a result, less space is available for devices. While these types of components generally only make up
a small part of the board population, they take up most of the set-up time because the operator or production engineer needs to program the component's data into the machine. Manufacturers whose markets include prototyping or small series productions, in particular for large original equipment manufacturers (OEMs), are finding that highly complex components with dense or complicated interconnections, such as for flip chips, are becoming more commonplace.
To remain competitive in the industry it is essential for the economies of production that these complex components are mounted accurately They are not only by far the most expensive components, but also the most time-consuming if rework should be required. Operators and production engineem need tools that make setups and changwvem both fast and accurate. Alignment and placement accuracy are paramount, and the complications associaied with handling even one complex, non-standard component on a board accurately can literaUy bring production to a standstill.
Being faced with new components is close to a daily occurrence for many operators and production engineers, and since each complex or new component is very expensive, the number of components supplied is often limited. As a subcontractor or OEM producing prototypes, it is likely that you will get the only two ever made, mahing accuracy even more important. It is not uncommon that these types of complex components are delivered to production without any computer-aided design (CAD) data. Where no package data is available, it needs to be created quickly at the machine. However, manual programming of package data for these complex components is neither quick nor easy
Asymmetric BGAs are diffcult and time-consuming to program manually since the coordinates of each ball in the array need to be entered. Very small components often require a microscope to see the details of the ballslleads. Larger components, too, can be challenging because they are difficult to illuminate effectively, commonly resulting in shadowing effects along the perimeter Low contrast between leads and the background male false detection of errors by the vision system common. This is particularly true of CCGAs with their almost mirror-like undersides and the 'busy' images of micro BGAs with traces of their interconnections showing through underneath.
SEEING THE UNDERSIDE T y p i d x it can take an experienced operator or production engineer anywhere from 30 minutes to a couple of hours to program a new component, depending on its complexity. Normally, this is done when setting up the machine for a new job for the fmt t h e , resulting in very long set-up t i e s
Placement error of increasingly complex components are threatening to neutralise their benefits
and machine downtime. While this costs time and therefore money the costs of faulty placement, which jeopardises the quality of the entire board, are potentiiy even higher.
So, what can be done to reduce both the time and risks involved in handling these delicate, expensive and complex new components? One method is a so-called 'auto-teach' solution that speeds up the programming of 'non-standmY or complex components such as those described above ~ providing a way of creating package data for the mounting process. If needed, this solution can even be used to 'teach the machine how to mount standard components.
The process starts with taking a greyscale h a g e of the component using the machine's vision camera. This image is then analysed by the computer to detect the ball grid array or leads. The software then uses a 'snap-to-grid function to match the measured data against a standard grid, after which adjustments can be made. The image result is presented to the operator on the screen, with each detected lead or ball shown and localised. Should there be a ball missing or a bent lead on the component from the manufacturer, or a fiducial mark on the lens that has been mistaken for a hall, the operator can use the software to add and/or remove leads o r balls directly on the screen.
The resultant output is component-specific CAD data, which is based on standard pitches. This is important so that slight variations in the individual component that has been used to 'teach' the machine do not have a consequential afect on the component CAD data.
Step two in the auto-teach function uses the vision software to set up illumination and vision tolerance parameters for the component, in what is essentially an
IEE Manufacturing Engineer I OctoberiNovember 2004
optimisation stage. The component is illuminated from different angles and the camera images are collated. Then the best illumination is selected, eliminating potential sources of error such as shadowing, and the function automatically determines the quality parameters for the component. The resulting package data can then be shared with other machines in the facility making it unnecessary for the procedure to be duplicated at each machine. The entire process takes only a few minutes.
MACHINE INTELLIGENCE Lets say for example, that the operator is faced with a new asymmetric BGA never mounted before. Rather than going through the timeconsuming process of manually entering the array, the auto-teach function can be used to rapidly teach the machine the BGA - how to align and place it and what illumination and tolerances to apply, The finer the pitch and smaller the component, the bigger the benefit, since fine pitch is exceptionally difficult to measure manually,
When each component is picked for mounting, it is inspected by the vision system and compared with the package data. Defective components, those that do not fall within the quality parameters set by the machine, can be identified and rejected before they are mounted on the board.
Importantly, not only is set-up time reduced, but new component types can be programmed into the machine at any time without loading any specific program or causing interruption to the production run. Operators can learn to use auto-teach very easily, thereby reducing the workload for production engineers, freeing up technical staff resources for more complex tasks.
Auto-teach functionality also eliminates the need for component drawings, which are needed for the manual programming of package data. The machine is also able to find min/max values for best performance better than an operator programming these manually, Should a change of vendor become necessary resulting in s l i t