Tooling is one of the most time-consuming and critical components of any product changeover or set-up. To many, the ideal tooling would configure itself automatically at the flick of a switch, without programming or hand placement. This is the story of how DEK set out to turn that ideal into reality.
The average PCB density has been predicted to exceed 176 components per square inch (27/cm2) in 2001, with an average of 54 pins per component (Source: AMRC, 1997). In terms of manufacturability, this means fewer suitable locations for tooling pins, thus compromising support for the assembly during processes such as screen printing. Such restrictions apply to all current types of reconfigurable tooling, and it is even becoming difficult to ensure satisfactory support using custom nests. The full custom approach is becoming more cost-effective at lower volumes thanks to computer-aided tooling generators, although prototyping, low production runs and board modifications are not easily accommodated.
Firm support for the assembly in planes x, y, z and q is the starting point for consistent paste volume and alignment during screen printing, and maintains the gasket seal between board and stencil. Clearly, a radically new solution is needed to restore support for the board during processing, and also to set manufacturers free from the time consuming task of identifying suitable tooling locations on the densely populated surface of a modern PCB. Currently, a tooling block can take up to 2-days to manually configure. Auto-placement of tooling pins is also complex, with inherent problems in the approach.
Until now, a founding principle of tooling set-up has been to align tooling pins with vacant real estate on the surface of the board, avoiding contact with components. But Bernward Starke, Engineering Manager at DEK says it is time to let go of traditional methods. "If this dependence on finding free areas of the PCB surface is not broken, then SMT processes will not keep pace with component technology and will ultimately fail to meet the demands of end-users."
Visualising the solution
Bernward Starke's engineering team set out to solve a problem the whole world knew was coming. Their approach led them to challenge conventional thinking in a number of engineering disciplines. "It was an exciting project," continues Starke. "We made it our remit to question everything that had gone before; every limitation, every received wisdom, everything anybody thought they knew about screen printing and engineering design. Nothing was sacred."
The Weymouth team visualised a powered tooling pin array that would automatically adjust to match the underside contours of any assembly, thereby relieving the requirement to restrict pin contact to unpopulated areas of the board. Spreading the load evenly throughout the pin array would allow direct contact with underside features such as passives and component interconnects. "The idea was to increase the number of pins actually in contact with a given area of the assembly, compared with conventional tooling, to reduce the average individual component contact pressure. This contact pressure is a function of squeegee pressure, or the system pressure if ProFlow is fitted," explains Starke. This ability to allow direct contact with tooling pins without damaging underside components would obviate the need to program or manually configure the tooling array for each new assembly, allowing operators to save valuable time in set-up and changeover. There would be further, secondary benefits by removing the need to store, maintain and retrieve tooling configuration data as part of the product build documentation.
The concept is actually quite straightforward. If it could be realised, the result would match the desires of end-users more closely than ever before; rapidly reconfigurable, easy-to-use tooling, independent of the characteristics of the assembly. "We added a number of important constraints," comments Starke. "The solution had to be cost-effective and reliable, therefore we precluded elaborate software or electronic controls from the outset. We also aimed to enable retrofitment to existing printing machines as easily as possible, to allow more users to access the benefits." The new tooling solution would be called FormFlex, a name that neatly sums up what the team had in mind.
First manifestations
The 5-bar pneumatic supply, standard equipment with almost all screen printers, offered a suitable drive for the powered pin array. "By using this existing machine service, we could satisfy our reliability, cost and retrofitting criteria, exploiting readily available pneumatic logic for sequencing and control." However, driving the pins directly by pneumatic pressure would not allow the tooling array to accommodate minor height variations from one assembly to another, which would make FormFlex impossible to use in a production situation. The team therefore conceived a fluid-filled system, to be driven by air pressure. This hybrid approach is not a common engineering solution. "FormFlex actually breaks new technological ground, as well as breaking with screen printing conventions," comments Starke. "We also developed a custom specification for the fluid, to ensure the pins would raise and retract smoothly when working with the printer's standard pneumatic supply. It has its own name: DEK Techneat."
The pin array exploits the principle of 'interconnected fluid-filled cylinders', enabling it to compensate for normal production variations. This is the one and only reason FormFlex can support directly on components in contrast to any other tooling system. "It is a bit like a waterbed; it gives firm support but conforms to any shape," explains Starke.
"The first generation solution featured a pneumatically driven piston. The friction present in the system prevented the pins operating correctly, so we redesigned the air-fluid interface for smooth and consistent pin operation." Experimentation led the team to specify an interface material with optimum flexibility, excellent sealing properties and long operational life.
Breaking with tradition
The pins are raised by pneumatic cylinders, and lowered by vacuum. "Another new and unconventional technique," comments Starke. "Working within the limits of the services available to a standard automatic screen printer, we showed that such things can be done."
The FormFlex assembly includes air valves that apply distributed pneumatic pressure to each FormFlex module. Pressure is applied to a unique flexible pneumatic-hydraulic interface, which causes the pins to rise. As the pins conform to the contour of the assembly, a cut-off valve within the array locks them in place by sealing the channel beneath the FormFlex pins at the point when full conformance is achieved. "The cut-off valve decouples the pins from the pneumatic supply, then locks them by sealing the hydraulic side of the system," explains Starke. "The system also includes a custom-designed flow restrictor and filter to provide extra tolerance of, and to protect the valve from, impurities in the fluid." A fibre-reinforced membrane creates the interface between fluid pressure and spring pressure, and it is the deflection of this membrane that opens and closes the valve.
The design of the sequencer module deploys a venturi vacuum ejector, which effectively reverses the pneumatic pressure of the air supply. Applying the vacuum lowers the FormFlex pins. Upon actuation of the FormFlex reset switch, located inside the printing machine, the vacuum ejector creates a vacuum that is applied to the tooling array. In each module this acts to return the hydraulic system to its original profile, thus releasing the pressure on the cut-off valve. The valve stem then returns under spring force and releases the fluid supporting the FormFlex pins, so that they slowly retract into the module. "We defied convention to maintain simplicity. And I am pleased with the result," comments Starke.
FormFlex in production
DEK outsources production of the pins, pin array assembly and cut-off valve to a nearby specialist. Here the components are manufactured and passed through ultrasonic cleaning before assembly, functional testing and final delivery to DEK. "We take delivery of calibrated FormFlex pin array modules, boxed and ready to ship to DEK customers who require extra modules, or to integrate with systems we are assembling in-house. As FormFlex is a modular system the customer can easily add extra modules to create a larger tooling array as required. It allows us to outsource the manufacture, testing and packaging of the modules and ship them to our own customers with every confidence that each new module will integrate directly with existing pin array assemblies the user may already have installed." Up to six tooling modules can be mounted on the tooling plate of the screen printer. This capacity is defined by the requirement for universal compatibility with machine tooling plates; the sequencer and manifold assemblies are thus also designed for a maximum of six FormFlex modules. The modules are retained magnetically, with the front unit located by a dowel to ensure the tooling array is correctly positioned relative to the machine front rail.
"In initial tests, and throughout development, FormFlex has shown its ability to offer a significantly faster, more convenient alternative to conventional tooling approaches," asserts Starke. The potential for yield enhancement is equally clear; data from beta testing demonstrates paste volume consistency improvements better than 20%.
"We proved that tooling can be efficient, reliable and cost-effective, while at the same time offering incredible flexibility and ensuring that surface mount manufacturing is no longer subject to component packaging and board density trends. It is a breakthrough concept, and we incorporated bold engineering thinking to make it happen. We steadily refined our ideas to create the production-ready tooling solution that FormFlex now represents."
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