solder paste

Pressure printing systems

Conventional stencil printing techniques have fundamental limitations as regards paste handling:

The volume of paste available for printing is limited, so frequent replenishment is necessary Paste is difficult to contain within the working area Once dispensed onto the stencil, paste is exposed to atmosphere, and deteriorates with time, so unused paste has to be discarded.

The result is that, over the last decade, there have been many attempts to produce a viable sealed-paste print system. These attempts have accelerated since 1997/8, as there has been substantial focus on such methods as solutions to the challenges of high production rate printing.

The most fundamental constraint for fast printing is that the hydrostatic pressure in the paste is determined by the squeegee speed, but there are other considerations:

The solder paste ‘roll’ volume is depleted as paste is deposited on boards, there are usually differences in roll diameter between the centre and edges of the squeegee, where the roll naturally tends to taper down, and the rheology of the rolling paste changes with time due to exposure to the atmosphere. The result of all three factors is that the fluid dynamics change, causing significant variations in transfer force with time and position on the board Conditioning (‘kneading’) is required in order to ensure consistent paste flow Greatest transfer force is at the squeegee/stencil interface, which creates the potential for coining the stencil and may lead to metal/flux segregation.

Approaches to pressure printing

Sealed head, pressure printing systems take subtly different approaches to the design challenges of:

Paste retention – how to make an effective seal to the stencil around the periphery of the paste chamber . ‘Conditioning’, to ensure correct paste flow by providing an equivalent to the ‘paste rolling’ action of the conventional squeegee Applying an even pressure to paste in the working area.

However, there are a number of features common to all systems:

They are totally enclosed, to prevent solder paste exposure to the atmosphere They use direct downward transfer pressure, to eliminate the need for squeegees The print area is defined and kept constant by wiper blades. These do not act as squeegees in any way, but merely contain the solder paste and keep the top side of the stencil clean These wiper blades (or ‘shear blades’) approach at an angle, rather than trailing as a squeegee does. At the correct attack angle, the paste can be sheared without scavenging the aperture. Although it looks as if the blade should dig into the stencil, in practice this design is perfectly satisfactory, and some even claim that such blades work well with stepped stencils The area of contact is the full width of the head rather than a line contact as with the squeegee The contact time between paste and stencil apertures is consistent because of the fixed distance between the wiper foils The paste is held behind the first point of contact (the leading wiper blade), so will only enter apertures after the best possible seal between stencil and board has been made. With squeegees, the paste is ahead of the line of applied pressure, so can enter apertures before the stencil is intimate contact with the board, increasing the potential for paste bleed.

DEK ProFlow DirEKt imaging

The first of these heads to be launched was the DEK ‘ProFlow DirEKt Imaging’ head shown schematically in Figure 13. Figure 13: The DEK ‘ProFlow DirEKt Imaging’ head Figure 20: The DEK ‘ProFlow DirEKt Imaging’ head

The paste is supplied in a cassette, conceptually like a printer toner cartridge. Multicore paste cassette system for pressure printing Multicore paste cassette system forpressure printing

The Multicore Direct Imaging System Cassette shown is that company’s implementation of what is an open standard, which anyone can adopt without licence payment. The cassette holds 1.25kg of paste (substantially more than a standard cartridge) and is emptied through the holes on its top surface, which are sealed with removable tape when supplied. The collapsible plastic pouch which forms the body is flexible, which allows for a degree of manual kneading of the paste if desired.

The cassette base-plate is an integral part of the paste conditioning system, the holes in it working in conjunction with those on the transfer head to create a meander path for paste conditioning.

During the print stroke, the paste is pressurised by a piston which acts directly on the top surface of the plastic pouch of the paste cassette, keeping the conditioning chamber constantly supplied. At the end of the print stroke, pressure is removed from the system.

Whilst travelling across the stencil, the trailing wiper foil of the paste retention system ‘scoops’ the paste from the stencil surface, keeping the stencil surface clean and also inducing a rolling motion within the conditioning chamber, to help maintain the paste in optimum condition for printing. MPM Rheopump

The MPM system uses standard paste cartridges. As the pump head moves across the stencil, pneumatic pressure and friction between paste and stencil combine to cause the paste to roll inside the paste chamber, as shown in Figure 14. The circular shape of the chamber enhances paste rolling and eliminates ‘dead spots’ in the head. Figure 14: Principle of the MPM Rheopump print head Figure 21: Principle of the MPM Rheopump print head

Paste is ‘pumped’ into the stencil aperture onto the circuit board pad, then sheared from the main body of paste by the trailing edge blade. The blades on this head are mounted at 45° and made of specially coated metal, using Permalex technology from Transition Automation.

The MPM head has closed-loop feedback from the paste chamber to control the pressure applied to the paste feed. Capacitative sensors prompt the operator to supply additional paste when the cartridge level is low.

As with DEK, considerable effort has been put into the design and materials of the ‘side dams’, which fill the spaces between the two blades. The challenge is to eliminate paste wastage and build up of paste at the sides. MPM side dams are now made of a composite polymer material with improved wear characteristics, and the side dam pocket has been relieved to conform better to the blades. At the same time the blade and backup bar have been repositioned to protect blades from being damaged and provide better blade support at their interface with the side dam.

A key enhancement in the MPM system has been the development of the Variable Volume Actuator, designed to stop paste bleeding out of the pump at the end of the print stroke. The sequence of operation is:

The board loads to stencil height The actuators engage, pushing into the chamber to reduce the overall volume. The four actuators fitted to the head are all coupled together Only when the actuators have engaged is the chamber pressure monitored by feedback from its transducer: if above the lower control limit, the print stroke proceeds; if below, the chamber is charged to the upper control limit, after which printing resumes. Note that the chamber pressure is controlled by programming the air pressure to the paste cartridges, and not by the actuators At the end of the print stroke, the actuators disengage/retract, increasing the chamber volume, depressurising the chamber and pulling paste back into the chamber, hence stopping any bleed The board is lowered from the stencil at programmed speed.

MPM Rheopump head with Variable Volume Actuator MPM Rheopump head with Variable Volume Actuator

You may notice that this is comparable to the technique used on some Archimedean dispense heads, where a small reverse action at the end of the stroke is used to stop droplet formation at the tip and possible ‘dribbling’. Some pressure printing practicalities

As with dispensing, care has to be taken in material selection – not all pastes benefit from an enclosed chamber! In fact, certain paste formulations will even harden over time, this cold welding of the particles possibly being caused by a combination of a chemical reaction and the paste being pressurized. Also, depending on the head design, the paste may roll faster in the chamber than with squeegee blades, causing some paste formulations to shear thin faster than normal. On the positive side, however, costly agents added to extend paste life on the stencil may no longer be necessary.

Priming is the equivalent of initially charging the stencil before first use. Typically this is carried out away from the printer, with a removable sole plate in place, and with the head inverted to ensure that any trapped air is evacuated. When the print orifice is completely full, the head is primed and can be inverted and positioned on the stencil. Then, as with squeegees, sealed heads may take two or three passes initially for optimum print settings to be achieved.

All the designs allow for removal and storage, so that the paste can be removed from the printer, resealed and stored under optimum conditions during intermittent printing.

Cleaning the head can present some difficulties, generally requiring mechanical paste removal followed by immersion in cleaning fluid. The ease of cleaning varies between designs: for example, the MPM head has a push rod which extrudes paste from the chamber.

With the traditional squeegee, different sizes of board are accommodated by selecting a squeegee and applying the appropriate extruded length and volume of paste from the cartridge so that the paste roll is around 25-50mm wider than the board. Where necessary, polymer squeegees can even be cut to length. With sealed paste systems, this becomes impracticable, so heads are supplied in different widths.

As with squeegees, a skim of paste on the surface may be left on any unsupported areas and there can be more side overlap between head and pattern, especially with fixed sizes of head. In addition, the paste contact area is much greater, with the result that there is a higher total downward force on the board. For these reasons, the board generally needs to be supported better and over a wider area than with squeegee printing. Where a rail system is used for holding the board, tooling has to support the print head at each end of its stroke.

DEK experience suggests that there are two differences between sealed paste systems and conventional squeegee printing.

Prints are slightly thinner, which will show in the SPC results and may require artwork redesign The system has a very definite process window: slow speeds lead to paste bleed because of the high pressure, whereas fast speeds lead to ‘wedging’, where material appears to have been pulled away from the aperture wall behind the squeegee.

Results from pressure printing

Tests have suggested that pressure printing can be carried out more repeatably at the higher print speed range of the paste. However, like printing with squeegee blades, the speed attainable is directly dependent upon the type of paste being used. Higher viscosity pastes may require lower print speeds and possibly higher print pressure.

Typically, improved filling pressure means that fine pitch parts can be printed at higher speed, giving higher throughput, although the rate remains material dependent. Faster cycling is also aided by:

A reduced requirement for under-screen cleaning, with less solder bleeding as a result of improved board/stencil gasketing Fast cold start-up and changeover times, because of the ease of changing paste cassette or cartridge Minimum operator interference, with no need to maintain paste levels.

Of the other advantages claimed by sealed paste systems over conventional squeegee printing, maintained solder paste quality and reduced paste wastage is the most substantial and immediately quantifiable benefit. Because the paste doesn’t come into contact with air, there is no drying out and crusting of paste on the stencil, and little waste during changeover, shift-change, clean-up and down-time. Although the actual performance will depend on batch sizes and printer idle time, scrap figures in the range 0.5% to 2% have been reported, as against as much as 30-50% for conventional squeegee printing.

Not only is there a direct cost saving, but the costs of disposing of hazardous waste are cut, and operator exposure to solder materials and solvents minimised. A valid point has also been made that the reduced amount of superfluous paste leads to a cleaner, safer printer and generally results in a reduced requirement for maintenance.

Other advantages include:

Improved stencil surface cleanliness and reduced stencil wear A wider process window. This results from the separation of the interaction between pressure and speed, and ensures that the printing process is more robust and not so susceptible to small changes in process parameters Consistent volume for through-hole print applications as a result of the direct downward transfer pressure. However, caution needs to be exercised: although simply increasing print pressure and decreasing head speed allows the hole to be filled more easily, too much paste can be forced through standard apertures Better filling of small apertures. DEK reported that for 0.5mm pitch QFP pads the volume of paste using the pressure head was approximately 15% greater than with conventional printing. This comes from better filling and (presumably) also better release. MPM report a well-defined good quality print on pads for a 0.3mm pitch QFP. Interestingly, both DEK and MPM report little difference between deposit volumes for NS and EW pads, suggesting that angled print heads will no longer be recommended for hard-to-print fine pitch QFPs.

MPM results at 0.3mm pitch MPM results at 0.3mm pitch Self Assessment Questions

What advantages would you expect to get if your assembly house announced that they had just purchased a pressure-printing system? And what changes might you as a designer need to make?.

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Author: Martin Tarr http://www.ami.ac.uk/courses/topics/0222_print/index.html

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We bought air dispenser guns that fit the cartriges. I can't remember where we got them - sorry. We built little aluminum "holsters" for the sides of the machines to put the guns in. The guns are hooked up to air with low pressure regulators. Works good for us.

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My group supports a couple hundred lines all over the world. Most of them use squeegees but a few use either the MPM Rheometric pump heads or the DEK Proflow heads. I absolutely hate the enclosed heads. Flux separation, compaction, and the print quality at fine pitches is awful. With the enclosed heads we get shorting at pitches that are fine with squeegees. The print definition with the enclosed heads are less brick shaped.

About 2/3 of our sites use SEMCO tubes (the rest using jars) and most of those use them with automatic dispensers. That system works fine and it's my preference.

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At a previous location we used the Semco cartridges. Occasionally the operator would have to scrape down the stencil and knead the paste back in. That would keep the paste on the edges from drying out. It was just standard procedure.

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