What's in a Squeegee Blade?
In the SMT world, a whole lot! But know the right attack angle, too.
If we compare the stencil printer to an automobile, squeegee blades are analogous to where the rubber meets the road. You would never consider putting low-performance tires on a high-performance sports car; that would diminish the purpose. The same idea applies to a stencil printer. To fully use the capability of a high-performance printer, choose the squeegee blade carefully. How do you do that? Hopefully this discussion will offer a good starting point in choosing the right blade for the right application.
In stencil printing, we see primarily two types of squeegee blade materials: metal and polyurethane. Polyurethane blades with a high durometer rating (90-110) have shown success in many applications, but as boards get denser and components get smaller, the only legitimate choice is to use a metal blade. The primary reason is that metal squeegee blades permit a more controlled and consistent print height across the entire board area, as compared to poly blades. Hence, this discussion is restricted to metal blades.
We are at a point where the line between SMT and semiconductor packaging is becoming blurry. Miniature components such as 01005 passives and 0.3 mm CSPs/BGAs demand the accuracy and precise deposition of solder paste volume, as do wafer bumping and other semiconductor processes. It is well known that stencil printing is a complex process, influenced by a number of variables that include hardware, software, materials and process factors. Squeegee blade assembly happens to be an element of printing that can have a significant effect on the print quality. Print quality is defined here by quantity of solder paste transferred, paste deposition profile, bridging and insufficients. Studies show most of the aforementioned qualities are affected by squeegee blade type and attack angle of the blade. Material hardness and surface characteristics not only affect deposition quality, but also affect the amount of paste wasted by adhering to the blade. The smoother the blade surface, the less the material will adhere to it. We also have seen, through experimental work, a blade hardness of approximately 75-80 on the Rockwell A scale is highly effective in regard to providing a clean stencil and desirable deflection angle. The effect of the squeegee blade’s attack angle will be discussed later.
Metal blade printing only has two print parameters that typically can be controlled: squeegee speed and downward squeegee pressure. The speed cannot be set so high that the paste does not roll as it moves across the stencil or so low that the print cycle time does not keep up with the manufacturing line. The blade pressure is usually set so that no paste remains on the stencil behind the squeegee. Excessive pressure will damage the stencil by either coining the image edges or breaking the fine webs between small pitch apertures. Higher pressures will also shear thin the paste to such an extent that the flux will separate from the metal and problems such as lack of tack at placement or solderability will occur down the assembly line. Figure 1 shows a typical printing process.
Contrary to popular belief, the paste does not fill the aperture until the paste bead has traveled at least 75% beyond the start of the aperture and fills from the front-to-back. Here the attack angle of the blade becomes critical.
Most squeegee blade assemblies are designed to provide a fixed contact angle between the blade and stencil. This contact angle changes as the print process begins due to application of print pressure. The angle between blade and stencil at the time of print process (with print pressure and speed in active mode) is known as the attack angle. This is the parameter to control to obtain optimum print quality. What is the optimum attack angle and how is it obtained? Clearly, this will depend on various process factors such as paste type, board finish, component type, etc. Studies show, on average, an attack angle of 50°-60° provides a more repeatable and accurate paste deposition compared to an attack angle of 30°-45°.
There are different ways to get to this attack angle. One option is to design a blade holder that can be fixed at a certain attack angle (Figure 2a). Another option is to induce this contact angle by manipulating the blade thickness. What actually controls the attack angle of the blade is the deflection of the blade under pressure. The thinner the blade material, the more compliant the blade becomes, which means that, with a fixed squeegee holder angle, a different contact angle is possible. Figure 2b shows this effect.
The figures demonstrate the influence of blade thickness on the ultimate attack angle. A thicker blade material remains rigid and maintains the fixed angle of the squeegee holder. Whereas the thinner blade (Figure 2b) provides a flexible contact angle (based on the blade type, thickness and squeegee pressure) by exhibiting more of a “leaf spring” effect during printing. This effect produces a better pumping action to fill an aperture, thereby permitting the paste to start filling an aperture earlier than the 75% beyond the beginning of an aperture.
What we can conclude, then, is there is more than one way to achieve the correct attack angle for a process. A process engineer should keep in mind there is no free lunch. While attack angle is important in providing a well formed, repeatable paste deposit, it should not come with the price tag of high print pressure and low print speed. A process engineer needs to fully understand the process needed to determine the correct squeegee blade assembly, and more specifically, the squeegee blade type.
Rita Mohanty, Ph.D., is director advanced development at Speedline Technologies (speedlinetech.com); rmohanty@speedlinetech.com.
