The challenge of successful 01005 feature solder printing has been well documented, driven by the continuing trend to miniaturize electronic assemblies. Successful implementation of this technology will demand critical improvements to the process, pushing beyond the limits of current technology. One solution, which involves a large capital expenditure, is to use two printers and two stencils: one printer with a thin stencil to print the miniature components, and a second in-line printer with a thicker stencil to cover the larger solder paste volume requirements. A second, and more economical, solution would require process optimization and a very tight process window. This means identifying exacting process parameters, including circuit board fabrication (pad designs, etc.), stencil (fabrication method, thickness and aperture size), solder paste(particle size, rheology and activator robustness), printing parameters (speed, separation, pressure, etc.), placement(pressure, speed and accuracy etc.) and reflow optimization(atmosphere and thermal profile). Improvements have been made over the years to most of the above process inputs. Various studies have evaluated fabrication methods; investigating, for example, electroform versus laser-cut and comparing the effect of stencil thickness. Stencil printing studies have regarded the effects of print speed, print pressure, and separation speed to optimize solder paste transfer efficiency (TE). However, one crucial area that has not been examined is the blade contact angle. In part, this is because printers cannot program a variable contact angle. Theory
Good quality in stencil printing ultimately meansdelivering the right amount of solder paste to the right place on the substrate. The final paste layer should be flat,with even thickness across the deposit and the correct shape(pattern resolution). SMT stencil printers have two blade angle parameters: a static contact angle and a dynamic attack angle (figure 3). The contact angle is a function of the blade holder, and is formed between the stencil and blade, achieving contact with the stencil with no force between them. 
The attack angle is a function of the contact angle, blade compliancy, print speed and paste rheology; it is the sum of the static and dynamic forces acting on the blade during the print stroke. Although the dynamic attack angle is complicated to determine, it is primarily established by the static contact angle. Earlier theoretical and applied research has demonstrated that solder paste rolling generates the downward force vector that fills the stencil aperture with paste. A fundamental requirement of solder paste printing is the generation of the correct amount of downward force in the paste roll to properly fill stencil apertures. Too little force, and the aperture will not fill properly; too much force and the result is a premature breakdown of the solder paste. Our work shows the effect that varying the contact angle has on aperture fill and release. Previously, the attack angle was the only blade angle that could be adjusted easily in production. The primary way of adjusting this angle is to increase or decrease the blade pressure, which in turn deflects the blade to a different angle. Using this method, limited adjustments can be made without adversely affecting the solder paste or the ability to wipe the top of the stencil clean. However, decoupling the attack angle from the blade pressure by changing the contact angle significantly improves the capability of the process. Our study explored the effects of modifying the blade angle. We examined two variables in our testing. First, we changed the applied angle in software, available on the YGP. Second, we adjusted the attack angle by modifying the applied blade pressure. Test methodology
We undertook a systematic structured DoE (Design of Experiment) to determine the effects of blade angle on print transfer efficiency. The two main factors were blade contact angle (45°, 55° and 65°) and print pressure (40N, 50N and 60N). For this experiment, we used Alpha Metals OM-338 CSP, an IPC type 4 (22 to 38 microns) lead free solder paste with a 4 mil foil and a YGP printer. Blade length was 350mm and separation speed was held constant at 7mm/sec over a distance of 2mm. For this test, we used a board and stencil with varying aperture sizes and spacing. All the test patterns used a 10 x 10 matrix of square apertures. These varied from 0.05mm by 0.50mm square, with 0.05mm spacing between the apertures (figure 4). These patterns were printed on a bare pad with ENIG finish. The test board was designed and patented by Research in Motion to yield both bridging and insufficient solder at the extremes, to allow for an objective measurement of the print quality. 
Paste deposit measurements down to 0.2mm were made with a GSI Lumonics 8200 3D inspection system. This was the smallest deposit that could be robustly measured by this machine. Additional visual inspection was done below the 0.2mm pad size to determine the smallest aperture and spacing that could be effectively printed at each of the test levels. To reduce variation across the test, we attempted to keep the solder paste roll the same diameter by adding a small amount (~5g) of solder paste every six boards. Six boards were printed for each condition, performing a vacuum wipe after the second board. Data was taken on board 5 and 6 of each run. A cycle time of approximately 40 seconds was maintained throughout the experiment. The data was replicated in random order to yield the results. We analyzed the data from only one print stroke direction (front to rear stroke) to eliminate another potential source of variance. Angle and pressure were varied in a full factorial experiment with a two replicates. The runs were randomized and coded to minimize the effect of random error. The experiment was set up as described in figure 5. 
From our experience with a large CEM customer, acceptable yields have been achieved for 01005s in production using 0.170mm square apertures with a 0.076mm stencil. This results in an area ratio of 0.56 [AR= w/4T, where w = the width]. In light of this, we looked closely at the apertures that were around this value. In this test, the 0.25mm, 0.20mm and 0.15mm square apertures yielded area ratios of 0.61, 0.49 and 0.37 respectively on a 0.101mm thick stencil. Results and discussions
The data was grouped by area ratio, discarding the data with less than 0.15mm spacing, as this was prone to bridging, discerned by visual examination. This left 7 sets of 100 data points for the 0.2mm up to the 0.5mm devices. A total of 176,400 solder paste deposits were measured. The data was evaluated for transfer efficiency (TE), or the percentage of the theoretical maximum volume for the aperture in question. We performed a visual inspection of pictures taken of all the patterns for the 0.15mm pads with 0.15mm spacing. These were labeled by run number only, in orderto minimize analysis bias. Each unique combination of two boards was compared for transfer efficiency. The boardwith the greater transfer efficiency was given a score of +1,and the other a score of -1. These results were then addedfor all the combinations to achieve a final score for eachboard. Data were then decoded and analyzed for the maineffects of pressure and contact angle (figure 6). Figures 7and 8 respectively show an example of the best and theworst transfer efficiencies. 
Conclusion
The tests led to four major conclusions: 1. Greater transfer efficiencies are obtained for the same area ratio by reducing the blade contact angle. 2. Increasing print pressure decreases the attack angle, but has a negative effect on transfer efficiency. 3. The best results were found at the lower limit of the DoE, although further testing is needed to determine if this is the true optimum. Experience shows that too low a pressure and too low an angle will cause solder paste to remain on the stencil, resulting in inconsistent and thicker prints. Another DoE is therefore in progress to further investigate the lower limit. 4. A lower area ratio can be used to print 01005s if the blade angle is optimized. This will allow designers to more efficiently place these components on a circuit board, or alternatively, allow SMT engineers to use a thicker stencilto print 01005s. Using a thicker stencil would in turn allowfor a larger range of components on the PCB with a singleprint process and/or widen the process window for adiverse component set on a specific PCB. Having the ability to program the contact angle, and vary it for different process requirements, is a valuable tool for the process engineer faced with increasingly difficult challenges in SMT. This fact eliminates the potential for set-up errors, and allows for angle changes for different conditions, such as after a stencil wipe, breaks, or paste dispense. This tool substantially refines the process for printing fine features. To further examine this, additional studies are planned to evaluate the effect of transfer efficiency while focusing on optimizing stencil thickness, evaluating type 3, 4 and 5 solder pastes, as well as optimizing pick-and-place and reflow processes. Once these studies are complete, we should be able to better characterize the overall 01005 process and have recommendations for an extensive range of process parameters. ------------------------------------------------------- Reducing process variation A robust solder paste printing process for 01005 components has been designed that avoids the traditional two printer solution. Having the ability to change blade contact angle as a process parameter greatly widens the process window for printing fine features. Furthermore, varying the angle as a function of print number after a trigger (under stencil wipe, pause in the process, first board printed in a batch, etc.) reduces the overall variationof the process from one board to another. 
Assembléon is introducing a new screen printer, the YGP, for short cycle-time applications that varies the squeegee contact angle in software, depositing the exact amount of solder needed by each component. The 3S (Swing Single Squeegee) head uses a servo-driven squeegee with variable attack angle whichimproves repletion (filling levels) as shown in figure 2. 
The YGP prints at high quality, especially with thinner stencil thicknesses (0.08 to 0.1mm), due to a much lower squeegee pressure required to fully clear the surface of the stencil. Additionally, the YGP works well with half etched stencils having steps from 0.030 to 0.050 mm. Overall cycle time is improved by performing stencil cleaning during PCB transport, using a fastwet/dry/vacuum wiper. ------------------------------------------------------- About the authors
George Babka and Scott Zerkle are based in Assembléon Americas, Alpharetta, GA, USA. FrankAndres, Rahul Raut and Westin Bent are located at Cookson Electronics Assembly Materials,South Plainfield, NJ, USA. Dave Connell is located at Research in Motion, Waterloo, Ontario,Canada. | 
