Contract manufacturer Flextronics' mixed technology basestation assemblies required a print process that could be performed at high speed while consuming an unusually high volume of paste, without loss of quality. DEK worked in close co-operation with the EMS provider in order to develop an innovative high capacity version of its existing ProFlow DirEKt imaging printhead for the purpose.
Being situated on a small, idyllic island in the Baltic just off the coast of Sweden is just the first unusual characteristic about Flextronics Visby. The unusual location also mirrors the nature of the products assembled here. For the telecoms specialist is not just another identikit subcontractor churning out tens of thousands of handsets; the primary products are GSM basestation boards. And that means a stiff engineering challenge.
Formerly owned by Ericsson, the Visby site was sold to Flextronics in 1999, and has an area of 32 700 m2 for low-to-medium volume manufacturing environment, featuring six surface mount manufacturing lines producing a major amount of boards. A good example of one of its volume output boards is a basestation assembly measuring 400 x 250 mm, mounted with 1500 SMT components with pitches down to 0,5 mm. It also has large apertures and several power planes. In addition the basestation board uses up to seven TH connectors, typically featuring three rows of 30 (or more) pins, which are soldered using a pin-in-paste process.
The complexity of the board, in conjunction with the pin-in-paste process means that the assembly demands an unusually high volume of paste. Whereas a typical SMT assembly might need around 3 g of paste per board rising to 5 g for a mixed technology product, this basestation board consumes up to 7g per print stroke. And the demand on the printer is compounded by the production rate. To maintain quality production Flextronics needed a machine that could cope with high speed printing, putting down large volumes of paste, without incurring any printing defects.
Application engineers at DEK's UK facility were confident they could meet this challenge. The company's long established enclosed printhead, ProFlow, had been proven in hundreds of installations across the globe, many producing similar products to Flextronics' basestation board. The EMS provider's need, however, proved to be at the upper edge of ProFlow's operating envelope. The solution required DEK not only to enhance the design of ProFlow, but also to develop novel new software to enable programmable ramping (up or down) across a print stroke, (pressures and speeds) and to produce an optimised stencil design.
Although there have inevitably been a few teething problems along the way due to the pioneering nature of this project, it has ultimately proved successful in allowing Flextronics to print the particular board in question in volume with a DirEKt imaging process, and retain all the now well-established advantages it offers over a traditional squeegee-based solution.
"We had to work as a team to resolve this customer's requirements," says DEK's David Foggie. "And such capabilities as our unique in-house stencil facility proved an enormous asset, because the design of the stencil was also crucial to the success of the project."
"We had many problems trying to print this board without incurring an unacceptably high level of rework," explains Johan Sundberg, a production responsible for optimising in-house processes at Flextronics Visby. "Some areas of the board tended to be starved of paste - and rework costs made the process economically unviable."
At the start of the project, Flextronics Visby approached DEK, and a competitor, to ask them to solve the problem but with the one basic condition: that the solution be based on closed printhead technology. "DEK soon emerged as the preferred supplier. The company worked very well alongside our engineers to provide an acceptable solution," continues Sundberg. "They produced a design that allowed us to do three things: apply paste that had been thoroughly 'rolled' inside the ProFlow head, put down a lot of paste quickly, and vary print pressure and speed across the stroke in order to optimise print performance."
The solution came in the form of DEK's brand new dual-chamber ProFlow printhead which exploits a design that allows it to deliver an unusually high volume of paste. Furthermore, to ensure that it is delivered to the board in a ready-to-print condition, within the head itself the print pressure is applied in a manner that allows the paste to 'roll' in a continuous manner, similar to the way in which it would on a squeegee.
High-demand printing of this nature requires development of printer, process and stencil. As a company that has its own in-house stencil design and manufacturing division, DEK was well placed to bring the three elements to bear on Flextronics' problem.
The pin-in-paste areas of the basestation board not only required the novel development of a dual chamber printhead, but also a specialised stencil. DEK's in-house stencil manufacturing application engineers worked closely with Flextronics' process engineers to produce a 5 thou thick, electroformed stencil that helped fully promote the benefits of the new printhead.
"With a conventional stencil we found that there was unacceptable bridging on the pin-in-paste areas," explains Sundberg. "Too much paste was being fed into the apertures during the fast print stroke. DEK's engineers cleverly redesigned the apertures, by, among other things, incorporating shielding bars into some of the apertures," he adds.
The shielding bars helped to restrict the flow of paste so that holes with varying diameters filled at an even rate. After extensive trials and several iterations of the stencil design, 30 thou holes were left open, 40 thou holes were crossed with 4 thou bars, and 45 thou holes were fitted with 8 thou bars.
By developing the dual chamber ProFlow head, in conjunction with a specialised electro-formed stencil and a robust process, DEK was able to create a solution for one of the most demanding printing challenges the electronics industry had ever faced. The new version illustrates perfectly how, by working with a customer on an actual production problem, the company is now able to offer a unique stencil printing solution to other manufacturers as and when they reach the thus far unparalleled level of board complexity and paste consumption experienced at Flextronics Visby.
"If DEK had not been able to develop a workable solution for our problem we would have had to revert back to squeegee printing," concludes Sundberg. "This would have made it slightly easier to control applied paste volume, but it would have introduced other problems such as inconsistent paste thickness on the critical pin-in-paste areas, large amounts of paste wastage, and problems with maintaining paste quality due to its exposure to the atmosphere. With the new dual chamber ProFlow head and stencil, however, we have a fully-automated, consistent and robust process."
Susequent to the above trials, Flextronics Visby has since ordered two DEK Infinity machines. "The DEK machines are performing fine on our factory floor, as we would have expected from our trials," concludes Sundberg. "The DEK ProFlow dual chamber head is currently the best print solution for our particular type of problem given the technical needs of the product and the commercial needs of the customer."
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