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Progress does not stop, especially in the world of electronics and its wide variety of applications. More and more functions are being crammed into ever smaller modules. To master these challenges on the path from SMD technology to the world of microelectronics, it is no longer enough to simply make the components smaller. Instead, engineers must analyze the interactions between materials and take them into account for their manufacturing processes. In order to achieve good manufacturability, all parties, starting with the designer, the PCB manufacturer, the printer-, stencil- and solder paste maker, as well as the pick and place equipment manufacturer and the reflow expert, should be consulted. Only common effort will ensure good quality in the finalproduct. PCB design and pad layout
In SMT assembly, typically NSMD (non-solder mask defined) pads are used. However, when it comes to small passive components some tend to use SMD (solder mask defined) pads. The different appearance of both pad types can be seen in figure 2. On the left, the NSMD pad with the typical opening of the solder mask is shown and on the right the SMD pad where the copper extends underneath the solder mask can be seen. Recently a combination of the two pad types, a so called semi-solder mask defined pad has been invented and it seems to have some advantages compared to the other two. When comparing these three pad types, the question is do they behave any differently during solder paste printing,placement and reflow? 
Looking into these processes, it was found out that all three pad types do not show a significant difference in how close two adjacent components (gap) can be arranged on a PCB without creating any electrical or manufacturing problems. Relative to PCB manufacturing, solder paste printing, and component placement, the following was discovered:
• For all three pad types the minimum gap should not be less than 150μm.
• The limiting factor for the board is the registration tolerance of the solder mask which today is around+/-50μm.
• For solder paste printing, it is important that theremaining stencil material between two openingshas enough mechanical strength so that no damageoccurs.
• The SMD or semi-SMD pad has the advantage thatthe stencil sits on top of the solder mask and thereforeseals securely.
• The solder mask also stabilizes the bottom layer ofthe solder balls of the printed solder paste layer. Thisresults in a better defined print. The solder mask, especially for such small components, has to be well-designed in order to reliably cover wetable areas like tracks because uncovered tracks can cause components to shift during reflow. Figure 3 shows an example of tracks between pads which are not separated by solder mask. Another design error occurs when the track width is only slightly reduced compared with the pad width. This allows the solder to flow between pads without any barrier during reflow. When the solder is in the liquid state, the surface tension of the molten solder results in a force that pulls components out of their initial position thus causing misalignment. In this case, if the solderable pad area is not defined or limited by solder mask, the movement ofsuch light weight components is uncontrolled. 
Ensuring that the solderable pad area is well defined by the pad design leads to the so called “self-alignment” of components which can compensate for minor shifts in the paste print and/or component placement. In figure 4, one can see the accurate placement of three 01005 components with 120μm gap. However, in this case there is too much solder paste due to the relatively thick stencil (80μm) used, and consequently the solder paste deposit on the pad isslightly smashed. 
In figure 5, one can see what had happened during reflow. When the solder liquefied, the components were pulled together out of their predetermined position. The result is a pile of components which electrically would still work, but would not be acceptable in any quality inspection. 
Figure 6 shows the effect when the solder mask opening is too large and portions of the tracks are uncovered. It can be seen that even single components will be pulled to the side where a track is connected onto the pad. If pad 1 of a component has a track connected to the opposite side, then the component on the track connecting into pad 2 will end up tilted after reflow. The above examples prove that good PCB design is essential for quality, therefore underlining the importance of design for manufacturability, especiallyfor miniaturized components. 
Solder paste application
The principle of solder paste stencil printing is that the paste, which is first filled in the opening (by the squeegee), has in the moment of deposition less adhesion to the sidewalls of the opening than to the surface of the pad. If the adhesion to the side walls exceeds that to the pad, thenno print is possible. Since the adhesion to the side walls is a function of their roughness, it is important to select thebest combination of material and manufacturing methods for the stencil. Area ratio, which can be calculated when the dimensions of the opening and the stencil thickness are known, is also critical. Most times, the stencil thickness and opening size are typically fixed so the only remaining infl uencing factors are the selection of the stencil material and the method of manufacturing. Therefore, it is imperative that one chooses a solder paste with good printing properties and a fine enough powder size. The best solder paste release properties are achieved with electroformed nickel stencils; however, the best overall print results are achieved with laser cut nickel stencils. This is due to the fact that for both stencil types, the nickel is electrochemically deposited, but for the electroformed stencil, the openings are created with a film based process which is less accurate then cutting the openings using laser. A rule of thumb is commonly applied when selecting the solder paste powder type - five balls of powder should fit through the smallest stencil opening side by side. One should therefore use at least a type 4 or even better, a type 5 paste. For good print results, it is also important to have a well supported PCB. If the PCB is very thin (below 0.5mm), a vacuum tooling should be used to flatten the PCB and support it from the bottom. It is also important that the stencil is wiped off often enough so that it is kept cleanunderneath at all times. Component placement
For the placement process, the following are preconditions for good component pick up:
• Good component quality (all dimensions/toleranceswithin specification)
• Good tape quality (all dimensions/tolerances withinspecification)
• Feeder unit with sufficient positioning accuracy andrepeatability
• Feeder unit with fiducial for exact positionrecognition Once the component is picked up, the vision recognition system must have a high enough resolution and accuracy in order to precisely calculate and correct its position. It is also very important that conditions (figure 8) which would result in defects later on (e.g. face down or bill boarded components) are detected and corrected immediately. 
In addition, a placement system must be able to verify that no components are lost on the way from the feeder to the placement position. Due to the extremely small nozzle tip for 01005 components, vacuum sensing is not a reliable method of detecting the component. Instead, a laser sensor should be used to detect absence/presence of the 01005 component during the pick and place process (figure 9). 
During touch down of the component onto the PCB, the placement force and the velocity of the z-axis need to be controlled. If the speed is too high, the solder paste gets into contact with the component and may splash. Additionally, if the force is too strong, the solder depot will be smashed as well. Force control is essential to avoid any mechanical damage to the component which is usually very sensitive and may break if forces bigger than approximately 2N are applied. Finally, during placement, the PCB needs to be well supported so that no vibrations cause placed componentsto move out of position. Reflow
Reflow soldering in nitrogen is recommended for 01005 components because the solder pastes used are usually of the fine-grained variety (type 4 and often type 5), causing them to oxidize more easily than traditional type 3 solder pastes. Although one should not depend on the self-centering effect during soldering, reflow soldering in nitrogen improves this effect because it generally improvesthe solder paste’s wetting characteristics. 
As with other components, the reflow profile for 01005s should comply with the usual criteria such as gradual warm up rate/tombstoning, and compliance with the maximum permissible temperature/time limits for components,substrates and solder pastes. 
Self-centering does work quite well for very small components even when lead free solder is used. Due to the fact that 01005 components weigh only 0.04mg the surface tension of lead free solder is sufficient to move them. Figure 11 shows an example of a test board before and after reflow. The components were shifted by approximately 100μm and are perfectly aligned after reflow. But this selfcentering- effect has limitations. The solderability of the pad, the component terminations and the solder paste has to be of sufficiently good quality, and the pad, the printed solder paste depot and the termination of the component have to overlap enough in order to avoid tombstoning. If all conditions are fulfilled, self-centering works best and is most reliable for a shift of up to 50μm. Since the selfcentering effect is not a process element and cannot be controlled, it should not be a factor in process planning. Rework
Rework is not recommended when dealing with 01005 components. This is due to the following facts:
• Even very fine soldering tips are too big for 01005 components.
• A safe mechanical contact to transfer the heat is virtually impossible.
• The danger of causing mechanical damage to the component or pad is quite high.
• The very small structures and components cannot beseen with the naked eye. While mass production rework of 01005s is not feasible, however, if occasionally one component needs to be reworked, it could be done by highly skilled workers, equipped with a microscope, a laminar hot air pencil, microtweezers and some flux. Conclusion
In general, the use of 01005 components translates to a more expensive product because of the higher costs for materials (components, PCB, solder paste, stencil, nitrogen (reflow)) and more accurate equipment. The layout and board design need to take under consideration all process steps needed to achieve a “manufacturable” product of suffi cient quality. Because of the narrower process window, 01005 components are limited to applications where the range of different components is not spread very wide. While 01005s will not totally replace the most common passive components like 0402s and 0603s, it will be used increasingly in products where miniaturization is the key driver e.g. sub-modules, sensors and medical devices like hearing aids. In those applications, their small size ensures that any mismatch of thermal expansion coefficient causes no damage when thermal stress is applied. However, designers have to keep in mind that in the pad design for 01005s, the adhesion of the copper pad to the FR4 material may become the weakest point especially when the packaging density is high and pad dimensions get smaller. So far the most difficult process step in developing and establishing the 01005 process in production has been solder paste application. This is not surprising given the fact that this process step has always contributed directly or indirectly to the most defects detected after reflow on surface mount boards. As a consequence of that, it makes sense to utilize AOI to find and optimize the process parameters when setting up the 01005 process. It is also a good idea toinspect the printed boards in running production. ----------------------------------------------------------- The 01005 challenge 
01005 components challenge all processes in the assembly sequence because of their size of 0.2mm by 0.4mm. They are almost invisible, at least for the “naked” eye, and extremely light weight (0.04mg). With those facts in mind it is easy to understand that the whole assembly process, but even more importantly, the materials and the layout of the PCBs must be designed for the use of these components. Once the PCB is created, the solder paste application is the next crucial step. Since this step contributes signifi cantly to the quality of the finished (reflowed) board, every effort should be made to ensure its success. For the placement of these “dust particle-like” components, correct and reliable pickup is key to a smooth placement process. ----------------------------------------------------------- Importance of cleanliness
The processing of 01005 components not only requires very good process knowledge and planning to design the board/product for good manufacturability, but it also requires that the production is done under controlled environmental conditions. It is not necessary to do it in a clean room, but it must be done under clean conditions. Also temperature and humidity should be controlled and kept constant. Figure 12 shows a board where a small fiber had fallen across some pads. This would create open solder joints or tombstones if it wasnot detected before reflow. 
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