Print quality can be improved through better control of solder volume and consistent stencil wiping.

Print speed, squeegee print pressure and solder paste type typically are not changed during a working process. These parameters are not changed because doing so would cause the process to become unstable and unpredictable. Yet one facet of the solder printing process that constantly changes is print pressure. Print pressure is the amount of force the solder paste exerts during the filling of the aperture.

Our measurements indicate that a change in the solder paste volume on the stencil is one of the largest contributors to dynamic print pressure. (Paste volume can be stated as the diameter of the roll, or the weight of the total amount of paste on the stencil.)

In most processes, a certain number of boards are printed, and then either solder paste dispensed in a bead or an operator manually applies paste onto the stencil. Either way, the amount of the paste on the stencil and therefore, print pressure, can change dramatically through the course of a shift of production.

Dispensing small amounts of solder paste frequently in the center of the existing paste roll permits the print pressure to remain virtually stable. This, in turn, removes a significant variable in the print process.

It has long been suspected that solder print definition and volume are affected by factors not readily changeable during the print process. Two of these factors are blade angle and solder paste volume on the stencil.

Blade angle. Squeegee blade angle is the angle formed between the stencil and the squeegee at a zero pressure condition. This has generally been a function of the fixed angle of the squeegee holder, or the print head itself. Some earlier stencil printer designs had the ability, within a narrow range, to mechanically change this angle during setup. This limited the operator to a single blade angle between changeovers. As a result, and because the blade angle was difficult to change, it typically was set at one angle and never changed. Printer OEMs experimented with offering blades of different angles, but this never caught on in practice. With few exceptions, each printer OEM settled on a single blade angle for its entire range of printers.

Solder volume on the stencil. The solder paste volume on the stencil has an impact on overall solder paste print quality. For example, excessive paste will get caught between the blades and impede the natural rolling action of the paste, resulting in insufficient paste depositions. Insufficient paste causes the material to slide across the stencil, instead of rolling. Efforts to minimize the potential for excessive or insufficient paste have focused mainly on the frequency and amount of solder paste dispensed via an automatic dispenser. For purposes of time, dispense cycles typically are programmed to be rather infrequent. The net effect is only to prevent either extreme: too much or too little solder paste on the stencil.

Enclosed print chambers have largely minimized these effects. However, these print heads have not been adopted as industry standard due to considerations such as cost, maintenance, ease of cleaning, and substrate size variations. Additionally, some older solder paste formulations, as well as all water-soluble pastes, are not suited for this process.

Previous Experimentation

Previously, the effects of blade angle and squeegee pressure have been explored as they relate to the effects on fine feature printing. In a study performed last year, we determined that:

1. Greater solder paste transfer efficiency can be obtained for the same aperture size (area ratio) by reducing the blade contact angle.
2. Increasing print pressure may decrease the blade angle, but has a negative effect on transfer efficiency.
3. If blade angle is optimized, a lower area ratio can be used to print 01005s and µBGAs, allowing the opportunity to use a thicker stencil to print these fine feature devices.

Test Methodology

Testing was performed at Yamaha’s Hamamatsu, Japan, laboratories. The goal was to determine the solder paste fill pressure changes when modifying the following parameters:

1. Solder paste volume (100 – 500 g).
2. Blade angle (45° – 65°).
3. Squeegee pressure (50 – 100N).
4. Squeegee speed (30 – 90 mm/sec).
Testing was done on a Yamaha YGP Printer with 350 mm long stainless steel blades. Blade angle was modified via the use of a servo-controlled angle adjustment motor integrated into the machine’s print head. The material used was Alpha Metals OM-325MS SAC 305 solder paste.

For these experiments, a stainless steel stencil was used. A pressure transducer was mounted into the stencil to measure the downward paste pressure into the stencil. This was used to approximate the fill pressure of the paste into an aperture during normal printing operations. One parameter was changed at a time in order to determine the individual effects on pressure.

Experiment Results

Solder paste volume. The paste volume experiment used:
1. A blade angle of 55°.
2. A squeegee speed of 30 mm/sec.
3. A squeegee pressure of 50N.
4. Paste volume was varied between 100 – 500 g.

Pressure was measured throughout the print stroke and graphed. Figure 1 shows several things:

1. As solder volume increases, the transducer feels the pressure of the paste over a longer period of time. This is expected due to the large diameter of the paste roll: The paste is over the transducer for a longer time.
2. As solder initially contacts the paste sensor (Figure 2, Point A), there is a small spike in pressure. Because the solder is rolling forward, this is expected, due to the downward force of the paste at the leading edge of the roll.
3. After the small spike, there is a drop in pressure as the weight of the paste is transfered to the transducer (Figure 2, Point B).
4. Near the point of contact of the squeegee with the stencil, pressure is highest (Figure 2, Point C). This is due to the shearing action of the paste at the blade tip. As the blade begins to pass the aperture, there is no place for the paste to go but up the squeegee face. The filled stencil aperture provides the reaction force necessary to push the paste up the face of the blade.
5. It is important to note that the applied force on the squeegee blade is not the sole reason for the pressure increase. If this were the case, there would not be a gradual buildup of the pressure to the peak.

Rather, there would be a spike in pressure equal and opposite to the pressure drop at the end of the stroke. Further, if the pressure spike was a result of the squeegee force, then all the peak pressures would occur at the same value at the point of contact of the squeegee with the sensor.
These effects were common in all testing performed. In addition, significant improvement was observed in the paste pressure with a large diameter solder roll (Table 1).

In a majority of operations, operators scoop solder paste onto the stencil without precisely measuring the amount applied. A few companies specify the amount of solder paste to be placed on the stencil at the beginning of a production run (i.e., one full 350 g jar), but most do not. Yet even these former companies fall short of full control of the volume of solder paste on the stencil when setting up the paste dispenser. Most paste dispense processes are suited to a larger volume of paste (75 – 150 g), and dispensed at infrequent intervals (every 40 – 50 prints). As a result, the roll size can still change significantly during the course of a shift.

As seen from the above data, a 100 g change in solder roll size will cause approximately a 7% change in maximum paste filling pressure. This is a significant change. It is enough to dramatically alter the solder paste print quality, especially on small apertures.

Attack angle. The attack angle experiment used:

1. A 300 g roll of solder paste.
2. Squeegee speed of 30 mm/sec.
3. Squeegee pressure of 50 N.
4. Varied angle from 45 – 65°.

Pressure was measured throughout the print stroke and graphed in Figure 3. Similar characteristics are diplayed in this experiment. The lower squeegee angles flatten the solder paste roll. As a result, the same volume of solder paste contacts a larger area of the stencil (Figure 4). This causes the pressure to be felt over a longer period of the stencil stroke. 

Additionally, the lower angles result in a much larger maximum pressure. This makes sense, as the smaller volume at the tip of the blade should cause a larger force, as paste is pushed upward along the face of the squeegee blade. The same amount of solder needs to travel through this smaller volume in the same amount of time, resulting in a higher paste flow rate for a lower angle. Table 2 shows the actual values. 

Squeegee pressure. The squeegee pressure experiment used:

1. A 300 g roll of solder paste.
2. Squeegee speed of 30 mm/sec.
3. An angle of 55°.
4. Varied pressure between 50 and 100 N.

Again, paste pressure was measured throughout the print stroke and graphed (Figure 5). The results of the two pressure conditions in this experiment proved to be almost identical. Table 3 shows the actual values. This is an interesting result, as it previously has been thought a larger amount of squeegee force will bend the blade more, causing a higher filling force, as seen in Experiment 2. From this result, we can deduce that the angle changes that result from an increased print pressure are very small. If we were to calculate a hypothetical angle based on the pressure changes:

The value is 54.38°, a slight 0.62° change in angle for double the squeegee force.

It is standard practice to use only the minimum blade pressure necessary to wipe clean the surface of the stencil. This has been used in the past as an attempt to prolong the life of metal blades by minimizing their wear. Considering these data, the additional squeegee force has no appreciable effect on paste pressure. This eliminates any remaining reason to increase blade pressure beyond the minimum required to clear the stencil surface.

Squeegee speed. The squeegee speed experiment used:

1. A 300 g roll of solder paste.
2. Squeegee angle of 55°.
3. Pressure of 50 N.
4. Varied speed from 30 – 90 mm/sec.

Paste pressure was measured throughout the print stroke and graphed (Figure 6). The results are as expected:

As speed increases, the time over which the transducer sees the pressure is reduced. This is because the paste travels over the sensor much more quickly at faster speeds.

As speed increases, the maximum pressure increases, which is seen in the actual values (Table 4). 

The results make sense, as it takes more force on the stencil to push the paste up the face of the blade at a higher speed to maintain the rolling action of the solder paste. The dramatic peak that forms at higher print speeds better illustrates what happens during high-speed printing. Typically, higher speed printing requires the solder paste to be “worked” or printed repeatedly to thin the paste down to a viscosity that permits it to roll.
A thicker paste will need a higher force to get it moving back up the face of the blade from the tip. The higher the pressure, the more likely the tip of blade will be pushed up by the solder paste, causing a loss of contact with the stencil. The result is seen as the paste hydroplaning, or streaking behind the blade after it passes over it.

Efforts to eliminate this problem in the past have centered on increasing the squeegee pressure. This prevents the blade from hydroplaning (up to a point, until the blade is bent too far from excessive pressure). However, Experiment 3 shows this does very little to increase the paste filling pressure. The result is that, even with the higher paste pressure, there is too little time that the pressure is applied to the aperture, resulting in insufficient filling.

Discussion. As the data show, the parameters with the greatest effect on paste pressure are (in descending order):

1. Squeegee speed.
2. Blade angle.
3. Solder paste amount.
4. Blade pressure.

As a result, it would seem a faster squeegee speed would result in the best aperture filling during the print process. However, experience shows it has the opposite effect: The potential for insufficient aperture fill, especially for larger apertures increases with increasing speeds. It makes more sense to look at the aperture filling potential as the total area below the curve of pressure versus time, or

In a printing operation, it still takes a finite amount of time to fill each aperture. Aperture filling can be improved by increasing the time spent over it or increasing the pressure. In this case, the parameters that have the greatest effects on filling potential are:

1. Blade angle.
2. Solder paste amount.
3. Squeegee speed.
4. Blade pressure.

Practical Applications

In a printing process, concerns vary from the potential for insufficient filling of the apertures (caused by low pressure and a small amount of time over the aperture) to the potential for bridging due to excessive pressure or time over the aperture. How can aperture filling be optimized for common printing processes?

1. Optimize blade angle for squeegee speed. In a significant number of applications, print speed is dictated by a cycle time requirement. As a result, there needs to be a way to improve the time that the paste spends over the hole. Reducing the blade angle will flatten the roll, increasing the contact area of the paste. This not only increases the maximum filling force, it also increases the amount of time the paste is over the hole.

From our studies, it appears increasing squeegee force has little, if any, consequence on the effective squeegee angle. Specifically, when the applied force of the squeegee is doubled and resultant paste pressure measured, there was only a very slight increase in the overall maximum pressure reading obtained, correlating to a theoretical angle change of less than 1°.

To significantly adjust the blade angle with traditional solder paste printers, the options are:

A mechanical modification to the squeegee head.
Employing a different angle squeegee blade holder.
Neither of these options provides much process flexibility on the production floor. Further, they significantly increase the potential for setup errors. Unless the machine is equipped with a means to change squeegee angle as a process parameter, there is no way to effect this change in the machine program.

2. Increase the size of the solder paste roll for higher squeegee speeds. For printers that lack the ability to change the blade angle, a secondary way to improve the pressure and time over the hole in a high-speed printer is to optimize the amount of solder paste on the stencil. A larger solder paste roll will have a higher applied pressure, as well as a longer time that the pressure is applied. Too large a roll size for a given speed applies too much pressure to the aperture for too long a time, increasing the potential for bridging of fine pitch devices.

So, how could the paste roll size on the stencil be optimized? As mentioned, there has been a quasi-control of this by focusing on the frequency and amount of solder paste dispensed via an automatic dispenser. However, it takes a significant amount of time to perform this dispense. As a result, paste dispense is typically infrequent, with possibly too much material dispensed each time.

True solder paste volume control can be accomplished in one of two ways: via an enclosed print head, or to dispense more accurately and more often. For an enclosed print head, the paste roll size is constant: It is the size of the paste chamber. The problem with this method is there is no way to improve the time spent over an aperture without changing the print speed. In a short cycle time application, this might pose a problem. The ability to precisely dispense a small amount of solder paste after every print or after a relatively small number of prints will keep the solder paste roll volume/diameter constant. Most printers lack this option due to the long time required to dispense or the relative inaccuracy of the dispenser. (Note: The printer used for these experiments had the ability to dispense every print cycle without an impact on cycle time.)

During many recent print test experiments, there has been a focus on starting every trial with a fixed amount of solder paste on the stencil. In this way, the results of the testing are much more consistent than what is normally found in production. This same logic should be applied to the production floor.

As the data show, there is a significant difference in paste filling pressure when solder volume on the stencil is varied. Since we know the printing process consumes solder paste from the stencil at a fixed rate (for a given board / stencil combination), we can calculate the usage over time. For instance, if 2 g of paste were used per print stroke, the operator could add 150 – 200 g of solder paste every 75 boards, or the dispenser could put down 75 – 100 g every 40 boards. In both cases, the amount is approximate, and over the course of a shift, the errors in the volume added to the stencil can add up over time.

The errors in an operator adding solder paste by hand are readily evident. Different operators will place different amounts of solder paste on the stencil. Further, the same operator will not place the same amount down every time. The errors in paste dispensing are also readily evident. Determining the exact amount of solder paste to be dispensed is not typically done when a board file is programmed. Instead, a single dispense profile is created, and then the same settings are applied to all the other programs. In these cases, volume errors multiply, but now they do it at a fixed rate.

Paste dispensers themselves are not very accurate. Most are reciprocating bulk dispensers that apply air pressure to the back of the stopper in the cartridge. The time it takes to pressurize the cartridge varies significantly through its usage, depending on how much paste remains in the tube. As a result, the actual amount of solder paste dispensed varies significantly from the first to the last dispense cycle. Finally, when dispensing a large amount of solder paste with little paste remaining in the tube, the full amount required may not be dispensed, adding further error to the total amount of solder left on the stencil.

Better-designed paste dispensers place paste in the center of the roll and dispense paste after only a few boards. The ability to adjust the required pressurization time depending on the percentage of material left in the tube minimizes the difference in the amount dispensed from the first to the last dispense in the tube. Because the dispenser does not move across the length of the paste roll, there is no cycle time penalty to dispense as often as necessary. Dispensing in the center of the roll also minimizes the tendency for paste to spread beyond the blade ends. This further minimizes the variation of the amount of material actually printed.

The ability to cut the paste at the tube opening after dispense ensures that all of the paste that leaves the tube drops onto the paste roll. An automatic shutter covers the tube outlet, ensuring no additional paste drops out of the tube in between dispense cycles.

3. Minimize the paste sticking to the blades. One other factor in the effective solder paste roll size is the amount of paste that sticks to the blade. On a two-blade system, the more paste that sticks to the first blade, the less paste that is available for the return print. As a result, it is not uncommon to see adequate print quality on one squeegee direction, but not in the other. Much of this effect is due to the solder paste characteristics and the blade finish. Some solder pastes tend to stick more to the blades at colder temperatures. Additionally, some squeegee blade finishes are more prone to material sticking.

The ideal process has a single squeegee blade that flips over the solder paste roll in between prints, keeping the paste on only one side of the blade. This minimizes the effect of paste sticking to the blades because the paste is available on the same side of the blade for the return print. Using a squeegee with a durable, low-friction surface finish minimizes this effect further.

Conclusions

A robust screen-printing process requires control of as many sources of variation as possible. Not all variables are easy to control, but it is helpful to understand (and target) the largest sources of variation.

The testing performed revealed:
1. Blade angle affects the overall time the paste spends over the aperture, as well as the maximum filling pressure. Both characteristics have a significant effect on the resultant aperture fill.
2. The volume of solder paste on the stencil has a similar, but slightly smaller effect on aperture fill.
3. In operations where a change in blade angle is not possible, the volume should be tightly controlled to control the overall aperture fill. This is most easily done with more frequent, more precise dispense cycles.
4. Increasing speed has the most dramatic effect on print pressure. Since these effects are felt on the aperture for a much shorter amount of time, the net result is that the potential for insufficient aperture fill (especially for large apertures) increases with increasing speeds.
5. The minimum squeegee pressure necessary to completely wipe clean the surface of the stencil should be used all the time, as squeegee force has little, if any, effect on the overall filling force. 

Acknowledgments

I would like to thank Kouichi Sumioka of Yamaha Motor Corp. for the stencil design used in this application, as well as for setting up the experiment and collecting the data. The results of his work were instrumental in his design of the more precise, stationary solder paste dispenser described in this article.

References
1. George Babka, Scott Zerkle, Frank Andres, et. al., “A New Angle on Printing,” Global SMT & Packaging, February 2009.

George Babka is multinational account manager at Assembléon America Inc. (assembleon.com); george.babka@philips.com.

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