08 October 2001 - Enclosed head printing gains support worldwide

Enclosed-head screen printing was introduced to the electronics assembly industry in 1997, and aimed to reduce materials wastage, minimise operator intervention and tighten process control.
Four years on, DEK's ProFlow DirEKt Imaging technology has been licensed by the majority of screen printing equipment manufacturers and is supported by solder paste producers worldwide.
Among the user community, early uncertainties surrounding paste compatibility and the practical extent of materials savings slowed adoption in the marketplace, but over 1000 ProFlow units are now in action around the world.
Now, further additions to the ProFlow product range are also enabling screen printing to perform a wider range of electronic manufacturing processes.
Technical analysis after the launch of ProFlow DirEKt Imaging confirmed a definite superiority over squeegee based printing in most surface mount applications.
Compared to the new technology the traditional squeegee was slow, relatively inconsistent and, importantly, meant large quantities of solder paste frequently dried on the stencil surface and had to be discarded.
Enclosed head printing aimed to make preplacement less messy by enclosing solder paste within a quick-change cassette.
This not only isolated the paste from ambient conditions but also made stencil cleaning easier and less frequent, and claimed to create a better gasket seal between the stencil and the board by decoupling contact pressure from the paste transfer force.
This would lead to improved paste volume consistency and reduced smearing.
Furthermore, the improved material control offered by the cassette and enclosed head highlighted ProFlow's potential to introduce screen printing in a wider variety of electronic manufacturing processes.
For example, the potential for screen printing to improve throughput and first pass yield in solder ball attachment and wafer bumping in the semiconductor packaging industry has long been understood, and ProFlow now offers to make that promise a reality.
Its materials handling advantages are also beneficial for depositing fluxes, thick film inks, encapsulants and adhesives across the electronic manufacturing spectrum.
DEK has now shipped over 1000 ProFlow units worldwide, with the majority deployed in north America and Europe.
According to DEK's David Foggie, who has been involved with ProFlow projects in Asia and Europe since 1998, the launch of ProFlow represented a significant step forward from printing with squeegees.
"The early adopters were those who were prepared to work with us to make the necessary process adjustments in order to realise the potential advantages.
Since 1997 we have made ProFlow easier to adopt, and it is now a mature product with a number of configuration options to suit customer applications".
According to Foggie, solder paste compatibility has been the single biggest issue in gaining customer acceptance.
Perhaps not surprisingly, manufacturing businesses are reluctant to experiment with a variety of solder pastes if the paste they have been using successfully with squeegees turns out not to produce such good results with ProFlow.
"We have built up a large database relating to solder paste compatibility.
We continue to benchmark pastes from all the major solder paste manufacturers, and can establish a satisfactory process without requiring the customer to change supplier.
We have built up this knowledge in conjunction with our international applications teams, who are able to quickly integrate ProFlow at any site worldwide".
The first customers felt that the potential benefits of moving to ProFlow, which include reduced operator contact with paste, higher throughput and isolation of paste from ambient were offset by the amount of paste left behind in the cassette.
"Changes to the cassette design to gain extra rigidity enabled more paste to be extracted before a cassette changeover became necessary", explains Foggie.
"With the redesigned paste low sensor, the amount of paste remaining in an 800g cassette has been reduced to approximately 47g by the time it is removed for replenishment.
This means that customers can consistently achieve between 4 and 8% paste wastage using the cassette-based ProFlow variant".
The optional rechargeable ProFlow transfer head offers further paste utilisation efficiency at the expense of increased operator intervention.
The transfer head is refilled pneumatically using a purpose-designed gun.
"Refilling the head requires a slightly higher level of skill than replacing the cassette.
Many customers prefer the easy to use cassette, and for this reason we have made it the default configuration".
When printing with squeegees, the squeegee pressure is primarily responsible for establishing the gasket seal between the stencil and the board, and also influences paste transfer.
In contrast, enclosed head printing allows an independent paste transfer pressure to be established, which is separate from the system pressure that maintains the gasket seal.
Paste transfer pressure and system pressure can thus be optimised independently, yielding greater paste volume repeatability and hence tighter process control.
So ProFlow has demonstrated its superiority over the squeegee in terms of process repeatability, and is also now delivering on the paste saving promises made at its launch, with a range of product variants and some tweaks to the design of the cassette and paste level sensor.
But other customers are looking to ProFlow for a solution to a wider range of processes.
With the steady decline in the use of through-hole technology, maintaining wave or hand soldering processes for a small number of through hole components such as connectors is no longer viable.
Currently there is great interest in the pin in paste approach, where plated through holes are filled with solder paste before insertion of the component, thus allowing the entire board to be reflowed.
To achieve a satisfactory pin in paste process, much larger volumes of paste must be transferred a requirement that has proved problematical for traditional screen printing techniques, including ProFlow.
In response to the increasing demand for pin in paste, DEK has developed a new dual chamber ProFlow option, designed to maintain a higher quantity of paste in optimum condition for printing.
Solder paste must be "worked" before printing, if it is to flow correctly and produce consistent results.
However, when depositing large quantities of paste, such as during a pin in paste operation, the available volume of conditioned paste is exhausted more quickly, requiring process parameters such as print speed to be adjusted to relieve the conditioning time constraints.
The dual chamber ProFlow head has been designed to allow large quantities of paste to be deposited at high speed.
"We worked with fluid dynamics specialists at a London university to model and optimise the flow of paste within the chamber, to keep a large body of paste in perfect condition for printing", explains Foggie.
He emphasises that keeping the paste moving is the main objective; areas in which paste is stationary are unavoidable, but here it is important the paste does not become compacted.
Using the FIDAP Fluid dynamic analysis package, a number of head designs, including the original single-chamber head, were modelled and evaluated to determine the optimum chamber profile.
"The internal contours of the chamber are smoothly curved to improve paste flow within the head".
The dual chamber head also includes an integral "nozzle" to optimise the flow of paste through the stencil apertures.
Large components such as heatsinks or RF shields, which require a large volume of solder and are usually attached by hand soldering, can also be processed satisfactorily, enabling manufacturers to achieve significant cost-down targets.
The dual chamber option is equally capable of printing a step and repeat pattern, such as a batch of identical SIMM memory cards.
Here, a large volume of paste is required to fill consecutive sets of identically oriented apertures.
PumpPrinting also requires a large volume of adhesive to be transferred to the board rapidly, and can be enhanced in the same way using the new head option.
The dual chamber head is designed to perform these operations at production speed.
It is generally true that broadening choice improves the prospect of achieving a satisfactory solution.
An intelligently conceived product range, offering configurations adjusted to a well defined set of processes, enables the vendor to tailor a recommendation to meet the end user's criteria.
For example, the ProFlow range now includes a selection of print head lengths at 150, 300, 350, 400, 450 and 500mm.
The choice of wipers (the blades that meet the stencil surface to retain the paste and establish a gasket seal between the stencil and board) includes coated titanium blades for optimum results with metal stencils, and mylar for use with more sensitive screens and plastic stencils for adhesive printing processes.
Furthermore, a choice of metal wipers is available, including a stepped profile for optimum flexibility combined with high rigidity at the contact edge to prevent scooping.
Uniform thickness wipers are also available, and all metal types are titanium nitrate coated for longevity.
Since 1997 the ProFlow range has grown to include single or dual chamber configurations, a choice of print lengths from 150 to 500mm, a selection of application-optimised wipers, and a choice of cassette or refillable heads.
Customer trials conducted using ProFlow in optimised enclosed head processes have demonstrated attractive material and cleaning overhead costs.
ProFlow has also enabled the semiconductor packaging industry to access the throughput and accuracy advantages of screen printing.
Packaging specialists have been searching for a high volume solution to wafer bumping, solder ball attach and encapsulation, which are fundamental to new generation packaging such as ball grid array (BGA), flip-chip and chip scale packages (CSP).
To summarise, many potential customers in the surface mount arena have believed the transition from the traditional squeegee to the enclosed head demands a high level of skill and applications knowledge.
In addressing these uncertainties, DEK's programme of mechanical enhancements, combined with its growing database of applications and process knowledge gained over the last 4 years, have evolved the technology and techniques to a point where the company says users can now very easily and successfully migrate to enclosed head printing.

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