How to Select the Best Stencil for SMT and Advanced IC Package Printing？
The stencil selection process can be confusing, particularly when creating a stencil for a new application. This tutorial, which covers stencils for SMT and advanced IC packaging applications, offers guidelines to assist users in stencil selection and print optimization.
This article discusses three basic stencil manufacturing tech-niques, explains the benefits of each type and details the effect of aperture shape and size with regard to paste release. How squeegee material choices and general paste-printing parameters affect the process are also reviewed.Although the general principles are the same for printing a fine-pitch PWB and ultra-fine-pitch wafer bumping, subtle differences can mean the difference between a successful result versus a struggle to achieve desired yields.There are three basic methods of man- ufacturing stencils. Although all three work well for most SMT or paste-print- ing applications, the production of ultra- fine-pitch devices on wafers can be a bit more challenging. Cost, additionally, is a contributing factor in the stencil selection and design process.
Tencil Manufacturing Common stencil manufacturing methods include chemical etching, laser cutting and electroforming.Chemically Etched stencils are created by passing thin, photo-imaged stainless steel plates though an FeCl bath to etchunprotected stainless steel material from both sides of the stencil . This method creates an hourglass-shaped aperture that, in fine-pitch applications, may cause problems with paste release, even though the aperture walls are smooth.Laser Cut stencils are created by using a laser to cut through the metal. The laser creates apertures, one at a time,with trapezoidal walls (the degree of taper may vary, if desired). These stencils are generally more expensive to manu- facture than a chemically etched stencil (based on aperture counts). The inner walls of the aperture can be electropol- ished to achieve a smooth finish.Remember, if you are creating a stencil with 200,000 apertures (for a wafer appli- cation) you might see an increase in sten- cil cost, since most stencil manufacturers charge by the aperture. The paste release characteristics with this method are very good for both SMT applications and wafer printing. Electroformed stencils are best suited for ultra-fine-pitch applications and wafer printing. The process consists of grow- ing Ni plating around a photoresist of the desired stencil image.The photoresist is removed when the desired thickness is reached and a thick- ness of 25µm is possible. An electroformed stencil for a typical wafer application can cost 5-10X less than a laser-cut stencil with the same number of apertures
The most common way to characterize an aperture’s ability to print well is its Aspect Ratio. This is the ratio between the width of the aperture compared to the thickness of the stencil. This ratio, however, is not an accurate way to deter- mine how well a BGA or CSP aperture will release paste.In this situation, the aperture has an equal length and width (usually a circu- lar aperture).
When this aperture style is employed, Area Ratio measurement should be employed. This method uses the ratio between the contact area of the board, the area of the aperture opening, and the surface area of the aperture (inner wall area). Assuming a circular aperture, the equation is:The user should try to maintain a minimum value for the Area Ratio to be 0.66.Another factor to consider whenprinting SMT devices is the solder paste mesh size. For all apertures, the user should try to maintain a 4:5 solder-Paste SelectionA Type 3 solder paste (-325 +500 mesh) is typically used for most fine-pitch SMT applications. This paste should be able to adequately print down to 0.020" pitch standard devices (0.010" x 0.065" aper- tures). The maximum size solder particle in this paste is 0.0017" in diameter,therefore, the minimum aperture width should be 0.0085".However, printing a circular or square aperture of this size requires a stencil thickness of 0.003" (see table). On occa- sion, this will only be two solder particles high. Therefore, to obtain the best and most repeatable volume of solder paste in a wafer bumping application, the par- ticles need to be more tightly packed, which requires a smaller mesh paste (Type 5 or Type 6). The user should try to maintain a 5:6 solder particle height deposit.When creating stencils for wafer- printing applications, a square aperture will yield better volumes than a round aperture (about 80-85% release). Also, there is a benefit to having enough mesh between the apertures to maximize both volume and spacing between the apertures.
In an SMT process, it is typical to underprint (create the stencil aperture slightly smaller than the pad being print- ed). Underprinting aids in gasketing and reduces bridging.
However, when printing paste onto wafers, it is often necessary to overprint the pad. During reflow, solder shrinks about 50 percent, so overprinting the pad will not generally cause a problem with bridging. Typical bumps using Type 5 paste are shown in Figure 5, before reflow, and in Figure 6, after reflow at 220°C. A 125mm wafer contained a total of 344 die measuring 200 mils by 200 mils. Each die has a total of 317 bumps arranged in a “full array” pattern of 10 mil pitch.The bump pads (UBM) are electroless Ni/Au construction on Al metallization with SiNi3 passivation. The pads are 4 mils in diameter.
Squeegee Blade Types Typically, in a standard PWB paste-print- ing application, a metal squeegee blade is used. Tests have shown that when print- ing a standard, fine-pitch application with device pitch down to as low as .012 and aperture sizes of .008, a metal squeegee blade is perfect. However, when printing wafers, with the pitch of the device as low as 150µm and aperture sizes as small as 80µm, a polypropylene 90 durometer, trailing edge squeegee blade is used.With such a fine mesh, there is poten- tial to damage the stencil with a metal squeegee. There is no measurable scav- enging or scooping of the apertures when using a polypropylene blade. This type of squeegee yields the best results in this application when optimized.scavenging the apertures.Too much print pressure may cause bleed-out of paste under the stencil causing bridging. Insufficient pressure may actually cause volumes to be higher than desired.Typical squeegee pressure for printing wafers ranges anywhere from 2-2.5 lbs. per linear inch of blade, as opposed to 1-1.5 lbs. per linear inch when printing SMT devices. This pressure increase on wafer printing is due, in part, to the need for “off contact printing” or “snapoff ” printing.
Optimizing Printing In an ultra-fine-pitch application, it is often necessary to sacrifice a few seconds of cycle time to ensure proper aperture fill. Print speed is dependent upon the thixotropic characteristics, or shear prop- erties, and paste viscosity.Squeegee speeds for printing wafers may range from 0.4 inches per second (ips) up to 1.0 ips, as opposed to SMT squeegee speeds that vary from 0.5 ips up to as fast as 8 ips. Squeegee pressure and speed are dependent on blade-to- board contact and the type of paste.Available high-speed pastes can be employed for most SMT printing appli- cations with a device pitch of .020" or greater and a minimum aperture size of.012". Print pressure should be optimized to clean the stencil without overfilling The separation speed should allow the paste to release from aperture walls without leaving much residue in the aperture or damaging/distorting the paste deposit.When adjusting print parameters for SMT devices, it is necessary to fine-tune board separation speeds and profiles to match the thixotropic characteristics of the paste. This method is typical in an SMT and CSP process.
However, when printing wafers, “con- tact printing” is difficult to achieve because the separation is usually too fast and damages the paste deposits. The difference in adhesion characteristics between the substrate and the paste are responsible for most difficulties. A snapoff print will usually solve the adhesion problem. Snapoff printing utilizes a space between the top of the substrate and the bottom of the stencil. This gap, in conjunction with optimum print speed and squeegee pressure, will allow the squeegee to press the stencil to the substrate as the force is applied and the squeegee traverses the stencil. While the squeegee passes across the stencil, depositing paste through the apertures, the tension in the stencil allows it to peel away from the substrate at the rate of the squeegee speed, leaving a uniform deposit. The snapoff may be as much as 0.100". A 0.070" snapoff is typical for most applications.Print pressure and speed, when print- ing with a snapoff (defined as the distance between the top of the substrate being printed and the bottom of the stencil at rest), will dictate how the substrate is sep- arated from the stencil.
Snapoff The speed at which the board separates from the stencil is especially crucial in SMT, CSP and fine-pitch applications Stencil Cleaning Most automatic stencil printers employ a fully programmable stencil cleaner that uses a vacuum and solvents in order to clean the bottom of the stencil. This is necessary to obtain the desired volume of paste. Adjust wipe frequency, speed, vacuum speed and solvent to match the application. Wiping after every print is not uncommon.
Summary Due to space constraints, this article barely touched on the many intricacies involved in the preparation of stencils for SMT and wafer bumping applications. The basic manufacturing techniques are chemical etching, laser cutting and electroforming.SMT paste printing, now a mature process, is typically less difficult than stencil prepa- ration for wafer bumping.