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Special nozzles and vacuum grippers help placement machines cope with diverse components.

The spectrum of component shapes and sizes has been expanding steadily for over 20 years. Passives now come in 01005 case sizes (nominally 0.4 x 0.2 mm) and the active spectrum has pitches down to 0.25 mm for some array types. The use of connectors has also continued to grow. Automated placement equipment has kept pace with this, although process windows are tightening.

Connectors come in all sorts of shapes and sizes and fulfill interconnection needs categorized broadly as:

• Board to board.

• Backplane.

• Memory systems.

• Battery connections.

• Zero insertion force (ZIF).

• Telecom.

In each of these groupings, technical developments are continuing and many are aimed at making connectors into true surface-mount components.

Backplane connectors are usually vertical and press-fit THT, yet there is a trend to convert some into large SMT components with locator lugs. These components need the special vision systems and grippers that are becoming more common today. Pitches are usually 1 mm or larger, yet some are as tight as 0.5 mm. As long as the tolerances of PCBs grow and connector pitches are +/-0.1 mm or better, they will be handled easily. Locator lugs provide mechanical strength during interconnection, and also consistent location during reflow.

ZIF connectors are often surface-mounted to prevent mechanical stresses during connection and are designed for multiple connects and re-connects. Some have lugs but most rely on the solder paste to provide strength and location. They often have pitches of 0.5 mm and are often right-angle connects so that the top of the body provides ample opportunity for a vacuum nozzle to make a good, secure pick-up. Even if a vertical ZIF is used, it should have a plastic vacuum cap. Most ZIFs are lightweight and few present problems to placement machines or relow ovens.

Memory systems require a vast array of differing connector types. These can be quite large, say 80 x 60 x 4 mm, and are often U-shaped. Many can be picked with a regular vacuum nozzle. But, some are too large and require unique vacuum activated mechanical grippers or special vacuum chucks (Figure 1). Pitches for these are heading below 0.5 mm in some cases but the larger ones tend to have locator lugs to avoid reflow issues. Provided the placement machine vision is selected, they can be handled, albeit with care and a reduced x-y-u axis movement.

At one time, most battery connections were made by clip systems. For modern mobile technologies, the battery connectors are akin to small passive components. Some are small modular units offering five-way connection in a 12 x 5 x 2.4 mm envelope and these are being superseded by devices mostly 1 mm thick with a 2.7 mm high stiffener at one end. All have a good pick-up area and are light (about 1 g).

Automation is the driving force behind making connectors SMT components. The other factor is the technical advantages. A good example: the growing trend toward pin-in-hole reflow (PIHR) to permit certain leaded devices to be assembled using SMT. Many connectors that are available only in leaded form can usefully be assembled this way.

Although today's placement machines can handle large components, connector sizes have not grown massively. This is probably because the number of interconnections needed has not changed much. Any interconnection, be it a mechanical connector or a soldered joint, carries an opportunity for failure and the design-for-life needs of today dictate that failure opportunities must be minimized. OEMs are also faced with the need to cram ever more functionality into smaller spaces, thus connectors have trended smaller because of tighter connection pitches.

Press-fit connectors are not suited to SMT processes although the move to PIHR and SMT connectors with support lugs has meant that some level of placement force must be attained for the system to work properly.

As some connectors deemed to be surface-mountable became larger, the placement system was asked to handle a large object safely and accurately and to carry much larger masses than before. The high mass of ceramic parts led to the introduction of vacuum nozzles to better handle the weight. Thus, the problems stemming from an odd-shaped plastic connector owe more to center-of-gravity than to mass (Figures 2 and 3).

The requirements of the solder paste deposition system, probably a stencil printer, do not change from component type to type (this includes connectors). A printer or dispenser designed to handle the normal range of SMDs will also be capable of handling the current crop of connectors. The only consideration is whether the PCB is a backplane. Some placement machines are designed to cope with large boards but the placement machine will have a much more rigorous job than other process elements. Categories for consideration are:

• Maximum heights of pickup and placement.

• Optimizing.

• Vision capability and package recognition.

• Feeders including third-party systems.

• Placement forces.

• Grippers including specialized and customized systems.

• Vacuum pick.

• Maximum weights and masses.

The height of the pick-up and the system's optimizing power need to be coordinated. The maximum height current placement systems will handle is 50 mm and the trend from component manufacturers will be to push for higher components if possible. Lifting a 50 mm high device over a previously placed one would require a z-axis travel in excess of 105 mm to allow for clearance, PCB warp and solder paste thickness. Such a travel could suffer from "run-out" - the tip deviation caused when the axis is fully extended and unsupported. Most placement systems therefore rely on a lower height specification, say up to 30 mm, but use their optimizing routines to "walk" tall components around each other rather than lifting them over each other. These optimizing systems will also generally enforce the placement of all smaller objects first so that taller ones can be moved in the most efficient manner. Long z-axis strokes also take longer to reach their limits slowing the entire placement system. As a result there are sound reasons for not pushing heights beyond 30 to 40 mm.

Machine vision systems need to be able to inspect tiny devices such as 01005s and huge connectors of 80 mm in length. Many standard vision systems cannot cope with such a range. As such, many machines group component sizes into camera resolution types, which means flexible machines have to have at least two cameras. The camera designed for the connector can be programmed to look at particular areas of the connector, therefore only the ends of the connector are assessed for placement accuracy. The center sections are assumed to follow by default. If a single camera can be used for all purposes and can also see a long connector in its entirety, then the machine becomes more efficient and flexible (Figure 4).

Large objects such as connectors often require matrix trays to present to the pick-up system but an increasing number of fine-pitch connectors can be fed from tape-and-reel packaging or even "surf tape." All flexible machines must therefore be able to handle trays, but how many? Trays are bulky and a multi-level tray feeder - at least 20 trays - is essential unless the assembler is lucky and has just one large, tray-fed device to worry about. This is increasingly rare as the number of tray-fed types increases per job.

Several manufacturers make special feeder systems that handle large objects such as connectors. If these feeders are not designed specifically for a type of placement machine, then the machine manufacturer must develop interfaces. With the increasing use of SMT connectors of varied shapes and sizes, it is more important today to be able to use different machine systems.

With the advent of PIHR and connectors with mechanical lug locators placement machines have to be capable of pushing components, under programmable force control, into holes or cavities in PCBs. With placement forces as high as 50 N, a rigid machine structure and a properly supported z-axis are essential.

A connector must have an exposed area for mating pins to meet. If the connector is a vertical type, there will be a number of vacuum leakage points. Fortunately, most connector manufacturers offer temporary flat plastic caps for the placement machine's vacuum nozzles, or the placement machine will have a vacuum-activated mechanical gripper. Such vacuum-activated grippers are special purpose units designed for a particular component. They are not standard and can be costly.

The larger the component, the more likely it will have a high mass. While early SMT components were about 15 g, today's can be 100 g or more. Some machines have variable vacuum and gripper technology for awkward shapes and weights. At one time, larger components and connectors would have been placed by dedicated odd-form placement machines but now "standard" placement machines have much more scope by equipping mount heads with a wide range of placement forces, tooling and gripper systems.

Amphenol (including the former Teradyne Connection Systems) has developed a BGA-based board-to-board connector that can offer up to 300 connections in a 60 x 20 mm area that relies entirely on BGA processing to make it work. It is fed in matrix trays and has plastic caps for vacuum suction. It uses standard vision algorithms for BGAs and is mechanically compliant to permit stress relief during manufacture. It offers promise for high density interconnection but also follows current rules on BGA processing. Since it can be as tall as 30 mm, its size presents challenges for vision systems and requires special nozzles for pick up (Figure 5).

HDI substrates can obviate some of the needs for handling connectors and their possible joint reliability issues, but connector manufacturers are seeking ways to increase reliability while also conforming with automated production techniques. Many of today's connectors are treated like any other SMT component and those with peculiar requirements - such as large size, weight or shape - are being developed with automated placement in mind. Simultaneously, the placement machine manufacturers are developing vacuum nozzles or grippers.

More components than ever can be processed in an automated SMT line. The challenge facing placement manufacturers is to try to combine the increasing spectrum into a single machine platform to minimize the need for a variety of different machine types.

Fredrik Moberg is manager, Mydata Automation AB (mydata.se); fredrik.moberg@mydata.se.

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