Section II: Stencil Printer Challenges as it Relates to Cleaning
a. Aperture Fill Release
The print material, which for this discussion is assumed to be a SMT solder paste, should be applied in a quantity that fully wets the squeegee blade in a bead size that is at least enough for the paste to “roll” during squeegee excursion, but not so large as to contact any part of the squeegee blade holder assembly. An ideal amount of paste is considered to have a bead diameter that measures 50% to 75% of the exposed squeegee blade height. The print speed and applied squeegee pressure should be set such that the squeegee will wipe the stencil clean while also consistently achieving a rapid paste roll response. Sliding, skidding, and/or hydroplaning paste bead roll will impede the opportunity for the paste to fill the stencil apertures. The rolling paste bead will shear thin and experience a natural viscosity reduction via its thixotropic material properties leading to enhanced solder particle mobility to fill stencil apertures under squeegee pressure.
Filling apertures completely is an important prerequisite towards the next challenge of emptying all those filled apertures in a consistently organized fashion to transfer individual paste deposits onto the printed circuit board pads. For best printing results, both the aperture fill and release processes should achieve containment the powder alloy and flux into tightly bound groups (i.e. solder paste deposits), resembling the template of individual apertures from which they were created that do not spill into each other on the printed circuit board. The “containment” principle is important as it relates to this IPC standard document’s focus on stencil cleaning aspects. One penalty of a poorly set up print process is more demand for stencil cleaning maintenance. Critical stencil design factors that influence printing results and stencil cleaning requirements are described next.
i. Gasket
The term “gasket” is often used to describe the ability of stencil apertures to contain solder paste within their individual boundaries during a fill and release printing cycle. More specifically, the aperture gasket is the region of stencil foil around its perimeter that physically contacts the printed circuit board pad. The illustration (Figure ?) identifies the stencil aperture gasket. An effective gasket essentially seals the apertures off from each another, containing the solder paste inside them and preventing alloy powder and flux from escaping.
The reality of a production printing process is that even good gaskets will degrade with continued uninterrupted printing and the process will require a pause in order to clean off any paste material accumulation interfering with the gasket. Designing for maximum gasket performance requires understanding the design of the printed circuit board and sizing the stencil apertures such that the aligned contact of the stencil to the printed circuit board achieves theoretically tightly sealed apertures. Solder mask defined pads and non-solder mask defined pads may have significantly different aperture gasketing levels comparing same the aperture sizes.
The topography of the circuit board can also severely hinder aperture gasketing potential. At the local level, silk screen legend labels, solder mask thickness, surface traces, and via holes all contribute to aperture gasketing degradation by reducing the surface flatness of the printed circuit board. At the global level, board twist and warpage can also introduce planarity discrepancies that weaken aperture gasketing potential. Similarly, stencil frame twist and bow can also upset aperture gasketing capability. Within the printer, the uniformity of stencil to board contact is paramount for achieving well sealed apertures and can be impaired by board and stencil handling components of the printer that do not achieve coplanarity control. Full “on-contact” printing using zero print gap between printed circuit board and stencil is recommended to achieve the optimum stencil aperture gasket condition.
ii. Area Ratio
As the separation sequence in the printer is initiated and the circuit board begins to decouple contact from the stencil, the expectation is that the paste inside filled stencil apertures will drop out and remain attached to the board. Luckily, the predictability of such a successful (or unsuccessful) outcome can be predicted based on calculating Area Ratios using known physical stencil aperture dimensions.
Area Ratio is defined as the stencil aperture open area divided by the stencil aperture wall area and represents a comparison of the attachment contact surface areas for solder paste residing in a stencil aperture during a print stroke (see Figure ?). The IPC Standard 7525B describes this parameter in high detail. The value of 0.66 is widely known to identify the threshold dividing high and low area ratios. Low area ratio designs will tend to produce solder paste deposits with lower and more statistically variable “transfer efficiency” compared to high area ratio designs.
Transfer efficiency describes the proportion of paste deposited on a circuit board pad relative to the total aperture capacity from which the solder paste deposit originated. So a transfer efficiency of 75% means that 25% of the paste still remains in the stencil aperture after separation. As electronic products continue scaling to smaller and more portable form factors, the stencils used to print such circuit boards trend toward lower area ratio designs. The consequence of this is reduced solder paste transfer efficiency and more frequent and aggressive utilization of automatic under stencil cleaning maintenance routines to maintain acceptable printing production yield.
iii. Interspacing
Designing stencils with sensibly large area ratio values is advantageous from a transfer efficiency perspective, however, high I/O density applications will also have limitations on the maximum aperture size. Using too large an aperture size can result in wet paste bridging defects. For today’s smallest pitch components (0.3mm pitch) it is advised to maintain an interspacing between apertures that is no smaller than the stencil thickness dimension. For coarsening pitch level, the minimum interspacing should scale progressively wider, while also adhering to area ratio rules, to assure bridge free and high paste transfer results. Under stencil wiping routines should be implemented to ensure bridged paste deposit defects to not form at a programmed frequency interval that also accomplishes the minimum penalty on printing cycle time.
iv. Design Guidelines (This Section is Blank Because Content Already Exists in Section a. i., a. ii., a. iii.)
b. Stencil Options
As stencil technology continues to evolve and improve, the user has a wider scope of stencil variations to choose from. The most common stencil options are introduced next. Cleaning aspects will be reviewed.
i. Technology
The three fundamental stencil manufacturing technologies that exist today are Chemical Etch, Laser Cut, and Electroform. Chemical Etch is a traditional method for creating stencil apertures that is not commonly used today for that purpose. However, this technique is still commonplace for etching stencil foils thinner to create step profiles necessary in applications that require multi-thickness foils. Stencil designs that have step profiles located on the squeegee side (top) of the stencil are expected to accumulate solder debris with use where squeegee contact is poor. As there is no known “on the printer” topside stencil cleaning option, the stencil will need to be physically removed from the printing machine to remove excessive paste residue accumulation. Stencil designs that have step profiles located on the contact side (bottom) of the stencil may also be sensitive to solder residue buildup as well as more frequent aperture clogging. Fortunately standard “on the printer” under stencil cleaning systems are available to wipe off excess solder contamination and clear blocked apertures. However, the under stencil cleaner wiping contact may not be optimal around the perimeter of bottom side step profiles as well inside step down recessed cavities.
Laser cutting and electroforming are the two primary methods used today for creating apertures on stencils. Laser cutting is the more popular option since it is more widely offered at typically lower cost and faster turn to the customer compared to electroforming. Well maintained modern fiber laser cutting machines are now capable to accurately cut tens of thousands of apertures per hour at high accuracy and repeatability. Electroformed stencils utilize the plating process to build up the metal foil, atom by atom, around a template of photo defined resist pillars. The fully plated foil will have aperture openings remaining where resist pillars are removed. When comparing the physical aperture characteristics between laser cut and electroformed stencils, there are a few differences to note.
i. The aperture side walls from laser cut apertures will typically be more tapered and a coarser texture compared to electroformed. The foil thickness and bottom side aperture perimeter topography will be more variable for an electroformed stencil compared to a laser cut stencil. While the tapered aperture wall cross section (compared to straight) has been reported to improve paste transfer, data to support such claims is scarce. Similarly, the assertion of smoother electroformed apertures improving solder paste transfer can also be challenged. The cleanability differences (for either “on the printer” under stencil wiping systems or “off the printer” dedicated cleaning machines) between laser cut and electroformed stencil technology is not a well documented comparison. Therefore both stencil manufacturing techniques are considered to have matched cleaning compatibilities.
ii. Foil Materials
Electroformed stencils are always nickel foils, but laser cut stencils can be either nickel or stainless steel materials. Electroformed nickel will typically have a smoother aperture wall texture than both laser cut nickel and laser cut stainless steel. The aperture wall texture differences between laser cut nickel and laser cut stainless steel cannot be fairly generalized here, as laser tool operating variables and parameters are considered to have stronger influence on aperture cut quality than material alone. In line with the previous stencil technology discussion, there is not enough consistently reported data to recommend one foil material type over another for cleaning compatibility. Notwithstanding, there is a new category of stainless steel foils emerging, generically classified as “fine grain” materials. These foils comprise a higher density microstructure that is stated to yield smoother laser cut aperture walls compared to conventional coarser grain stainless steel materials. The alleged benefit of fine grain materials is improved solder paste transfer efficiency, and with supporting data, it is assumed this should also translate to reduced “on the printer” under stencil cleaning demand.
iii. Frames
There are many variations of stencil frame types available. The two common categories of frame types are “mesh mounted” and “mesh less”. A mesh mount frame format consists of a stretched mesh material, made of either polyester or stainless steel that is glued to a rigid outer frame. A thin metal stencil foil is glued around its borders onto the center of the stretched mesh. The center of the mesh is then cut out after the foil attachment glue is cured. Mesh less stencils are newer technology where individual loose foils can be directly attached to a common rigid master frame. In these systems the mechanical attachment process will generate foil tension by pulling perpendicularly outwards along the four foil edges. In addition to these two frame categories frames are offered in different sizes, materials, and thickness.
From a stencil cleaning perspective it is important that the frame be straight and level. Twisted, bowed, and warped frames will cause the stencil foil to follow this non planar profile, leading to greater risk of poorly gasketed apertures on the printed circuit board substrate and increasing “on the printer” under stencil cleaning demand. The manufacture of the stencil frame also influences stencil foil tension. High tension stencil foils may potentially improve paste release uniformity characteristics via enhanced z-axis rigidity that allows the foil to maintain a more stable position throughout the circuit board separation process following the print stroke. Any stencil deformation during board separation can inflate the possibility to produce variable solder paste volume deposits, necessitating more frequent use of “on the printer” under stencil cleaning.
The preservation of foil tension through the life of the stencil is another factor that can be different between mesh mount and mesh less frames. “Mesh mounted” foils are permanently fixed in tension, and remain so while subjected to “off the stencil” dedicated washing machine cleaning cycles. Such wash cycles can use chemistry and temperature that leads to eventual foil tension relaxation. On the contrary, “mesh less” frame types only subject the foil to temporary tension while it is mounted during a printing process. Foils in these systems typically require removal from the master frame while being cleaned in a dedicated washing machine. The absence of glue and mesh in a “mesh less” foil makes these less sensitive to experiencing natural tension loss. Frame size can be a concern if the squeegees required to print the board cannot comfortably fit and move unobstructedly across the entire clamped board width. The other caution with using small stencil frame sizes is to ensure both the board clamps and the under stencil cleaner are appropriately sized to accommodate that frame.
iv. Image Position
It is recommended when possible to position the stencil aperture artwork centrally on the foil. This ensures a balanced foil tension result is achieved across the aperture image, which can help paste release uniformity. A centrally positioned aperture image also allows the widest printable margin, which can be important for pastes requiring longer distances to generate “roll”. Similarly, the printing machine’s under stencil cleaner will also be allowed to wipe across widest area surrounding stencil images using center justified apertures. The multi-image stencil design strategy is the other option that is common in production, whereby one stencil foil can have many aperture images for printing the same or different substrates. Multi-image stencils only make sense with substrate designs small enough to fit on the foil without overcrowding each other. The incentive to use multi-image stencils is to improve stencil productivity, reduce overall stencil expenses, and lower stencil storage space. Pros and cons of multi-image stencil design strategy should be carefully considered.
v. Aperture Geometry
The shape of the stencil aperture typically matches the geometry of the circuit board pad and is slightly undersized relative to it, in order to accomplish an effective “gasket” condition (Section i.). Varying the stencil aperture geometry in any way that diminishes the opportunity to form a fully gasketed outcome is discouraged. There is elevated risk to contaminate the bottom side of the stencil with solder particles and flux using aperture designs that gasket to the substrate poorly. For example, use of square apertures (instead of circle) on round pads may result in imperfect gasket conditions around the aperture corners. Apertures with right angled corners, such as for QFPs, could be modified by rounding corners without hindering the resultant gasket. The logic for rounding aperture corners is to lessen the opportunity for trapping solder balls in apertures, and thereby reducing “on the printer” under stencil cleaning demand. Although this theory lacks definitive industry proof, there appears no significant negative consequence to round aperture corners.