Stencil Printing
The most important trade show for the electronic production industry takes place only every two years. Each time, Productronica in Munich is the stage where new technical solutions are presented – it is indeed the ideal setting to present innovations to a large audience of specialists. For that reason, the show held in 2015 was the ideal platform for Ersa’s “Festival of Innovations” – with a host of new features for existing equipment, but also for the presentation of completely new solutions. Amongst these new solutions could be found the VERSAPRINT S1-3D, the first stencil printer with integrated 3D inspection.
What had started at Ersa eight years ago as 100% inspection has now progressed to the next level of innovation. For all those customers, for whom floor space, flexibility, cost and an innovative partner are of importance, the VERSAPRINT SPI is the perfect solution. Presently, 3D-SPI is the established method to determine the print quality in SMT manufacturing. These systems are being used – in fact they must be used to ensure the quality of the assemblies – in the automotive industry and in all other areas where safety related board assemblies are being produced. So it should come as no surprise that a large number of suppliers have entered this market, offering a number of different models, complicating the decision making process for a user when sourcing a suitable system. Through intelligent software solutions such as a detailed presentation of SPC data – extensively graphically formatted – all systems attempt to satisfy the demands of the market and to distinguish themselves from the competition. But taking a closer look at these systems, it turns out that they are basically no more than mechanically simple machines, equipped with an axis system to move the inspection head, and a conveyor system for handling and clamping the board assembly. Systems, that are supplemented by a variety of different methods to capture the raw 3D data, which are based either on laser triangulation or white-light projection, two basically different concepts to measure solder depositions.
In laser triangulation, a laser beam is being projected on to the object to be measured. The light reflected from the object is being displayed at a triangulation angle on the sensor of a camera, and from the geometry of the optical structure the height information is calculated. Acquisition is made by traversing the measuring device across the PCB board and scanning the laser profile.
During white light fringe projection, also called Moiré, a full area illumination of the object to be measured takes place by a fringe projector. The projector projects, under a triangulation angle, a pattern of stripes on to the object to be measured, with an intensity distribution dependent on its particular location. This data is collected and interpreted by a full area detector. Acquisition of the topographical information is thus in a stationary mode. The main difference between these two process methods therefore: Laser triangulation takes place in a scanning process, while fringe projection takes place in a stationary mode. Common to both is the acquisition of the height profile through the angle of reflection of the light. So it is no surprise that there is no difference in the accuracy of the data acquired, and that therefore both systems are suitable to take on and assess the current printing process and its demands.
The mode of operation is the same for both systems. They are placed in an SMT line, separated by a conveyor from or by directly following a stencil printer. The next board, or the board following the next board can be inspected after the print, and a statement can be made as to its quality. If the cycle time permits, printing the next board could be waited with, in order to avoid defective prints. If the result has been established and the print is without defect, the board will be released and moved on to the next production step – or, if a defect has been detected, it is transferred out from the line. The disadvantage of this concept is clear: to an SMT production line, a downstream inspection system to monitor the quality of the core process stencil printing is being added. This consists of a completely independent system, build on its own platform and with its hardware as well as software. On the hardware side there is an investment to be made, and the floor space required and the maintenance costs need also be considered. On the software side, considerable training and process time needs to be spent if the system, to be optimally used, is to be adapted to the user’s specific processes and requirements.
Once the result of the inspection is available and if the printing process has been shown to be faulty, intervention by the operator is called for.
This is necessary, since process parameters such as squeegee pressure or squeegee speed cannot be automatically optimized, based on the defects found, by the inspection system. Defects such as excessive paste deposits or incomplete printing will be recognized, but not their causes. For this, the process engineer is called for.
Possibilities to intervene in the process through the inspection system are very limited, and intervention is realized through a closed loop interface. A detected print offset can automatically be corrected by the system for the next print cycle. More difficult will it be to initiate, based on changes detected in the inspection result, a stencil cleaning cycle. An increase in the thickness of the wet film after a number of print cycles is considered to be due to a deposition of solder paste on the underside of the stencil. These two inspection results are presently the only meaningful possibility to actively intervene in the printing process. Driven by the growing demands of the market, and in order to satisfy the general demand for an increase in quality, Ersa decided to get involved in 3D solder paste inspection, abbreviated SPI. Getting involved, while staying true to Ersa’s philosophy which stipulates: to be a driver of innovation and a pioneer – not to be a “me too”, but to come up with its own ideas and concepts. Motivated by the wishes and the vocal requests of our customers, it was decided rather quickly that we should adhere to the original concept of the VERSAPRINT model line – which is integration of 100% 2D inspection – and follow this up with integrating the 3rd dimension. Quite often, the compact design of the system, the integration of the software of the printer and the inspection function in a single unit was being praised. And once this decision had been made, everything went very fast: As the measuring method laser triangulation was selected because of space availability.
The design and functionality of the camera should also be able to, aside from measuring profile heights, recognize fiducials and the orientation of the board. With those demands, it became rather tight in the restricted area of the enclosure. For a safe and repeatable capture of the many different fiducial geometries and situations, both direct and diffuse illuminations are required. A solution to this turned out to be an elaborate prism to divide the beam and an intelligent camera, whose sensor chip could be divided in a number of segments. The resolution of the actually selected constellation is one of 17 µm in the lateral and scanning direction and less than 1 µm for the height measurement. The system is therefore in the position, to gauge the presently smallest components respectively their pad areas or solder depots. The scanning speed of the camera is currently 140 mm/s at a scan width of 34 mm, meaning that appr. 15 sec’s are needed for the inspection of a double-euro-format board 220 x 160 mm. In order to satisfy the demands of the customers with regards to the cycle time, both models of the Ersa printer line are available. The S1-3D model processes the board sequentially, and the inspection takes place following the printing sequence. The P1-3D, on the other hand, allows for parallel processes, meaning that while one board is being printed, the board having been printed just before is already being inspected in the exit zone of the system.
In 3D-inspection, the following properties are assessed: volume, area, height, shorts and offsets. The Gerber file of the stencil is used as reference for the surface area – so that only the thickness of the stencil has to be added manually by the operator. If it should be necessary that some areas be evaluated differently, such as for example for step stencils or for differing process boundaries, can this be done through freely defining certain areas within the layout. These areas can be added at all times to the existing program, thus leading to a steady optimization of the production. Since the range of the individual pad areas to be searched is larger than the nominal surface, it is possible to not only identify a condition of insufficient, but also an excessive amount of paste. At the same time, and almost as a waste product, a possible print offset will be recognized, since the solder depot detected is not only assessed in relation to its nominal area, but also to its nominal position. For an offset and its correction during the running process it is typically not the full value determined which will be corrected, but only a percentage value of it. Since by neglecting the outlier values, the optimal offset values can and are approximately being determined. In the normal case, the average offset value of all pads is being considered. However, it is possible to define up to two areas, so that the weighting of the position can be defined freely within the layout. This helps with prioritizing critical areas on stretched printed circuit boards.
The recognition of shorts with 3D inspection is rather problematic. If the height profile of the laser line is used as a mark, it would be necessary to first define a minimal height of any eventually present paste. But then elevations of the solder resist due to traces or vias would be recognized as possible bridges, pseudo defects would be the logical consequence. A 2D measurement of these areas, on the other hand, allows for a reliable analysis of the areas between the pads. This technology has been optimized by Ersa for their 2D inspection process, and this is the reason why it is being maintained for this new development as well. A parallel, simultaneous 2D and 3D inspection would technically be possible – but since the clock rate of the camera would need to be split between both modes of operation, this feature has been dismissed for obvious time reasons. In its place, the 2D image of the laser line is analyzed, where the presence of paste on the solder resist is excellently shown.
Always an important topic in 3D inspection is the definition of the zero point for the height measurement. What is meant here is the height of the unprinted pad. Since these pads normally are, after the printing process, no longer visible, this height has to be brought into relation to reference points in their vicinity. One frequently used method is to measure unprinted boards as references for the following batch. For this, boards are fed into the SPI system and the height of the unprinted pad is brought in relation to the surrounding solder mask. This is a rather complex method, which needs to be repeated every time when the height of the solder mask has changed. This will happen when the board supplier has changed, or simply when a new batch of boards is being started. Here lies an obvious advantage of inspection integrated in the printer: Each board can be measured, over a frequency and a counter, just prior to printing. After the counter has expired, pre-inspection is suspended and activated again only if some facts have changed. These can be fluctuations in the batch, which are compensated for through regular periodical measurements. For very high demands on the accuracy of the results or if the cycle time allows, the system can permanently run the pre-inspection process.
Another method to define the zero-point of the pad is the use of local fiducials or test points., Demanding boards, in particular, have very often fiducial marks which are used by the pick-and-place system in order to further increase their accuracy for placing fine-pitch IC’s. Or they have in-circuit test points for a subsequent functional test available. Those surfaces, distributed across the printed surface board, can ideally be used for defining the zero-point, since they are on the same level as the pads. This method of measuring the zero-point has the decisive advantage that variations in the thickness of the solder mask are of no relevance, and a pre-inspection can be dropped. These points are learned automatically through a scan of the printed board or by manual definition by an operator. Inspection is performed exclusively by laser triangulation, and only when a defect is detected will an additional 2D image be taken, for a better presentation and analysis for and by the operator. The 3D image can be rotated in any direction and allows the operator a quick and safe analysis. Height data are highlighted by color and their border areas are coded yellow and red. Additional modes of presentation such as cross sectional views or plan views are also available, giving the viewer different types of views and viewing angles with which to validate the inspection results. Aside from the absolute limits of inspection – min/max volume, min/max cover, height, short, offset – there are also “Alert”-limits. They early on alert the operator about a trend, allowing him to take corrective measures. For example, an automatic stencil cleaning cycle can be activated, or a stencil inspection with the aim to improve valuation.
Stencil inspection as a feature of 2D inspection remains in place as an excellent means to determine the need for a stencil cleaning cycle, should the smearing on the underside or a blockage of the stencil exceed the preset limit.
SPC data acquisition and analysis is a favorite feature of SPI systems. Ersa therefore offers a software package to process the acquired raw data. With it, offset values are portrayed as Gaussian curves, Cpk-values are determined and trends for product batches as well as complete production shifts and days will be analyzed. The result of the inspection can be recorded as error images, or as error-free circuit boards. This enables the user to subsequently qualify and analyze the results whenever called for. The software itself can be used either immediately on the system to optimize the process, or at the end of the line as comparison with the AOI system or, finally, as a control tool for production management.
3D-SPI will assume an ever growing importance in today’s SMT production lines, answering the call for increasing demands on quality. The integration of stencil printing and 100% 3D inspection into one system represents a level of automation previously unknown in the process step stencil printing, thereby achieving, independent of the operator, a consistent and repeatable quality of the process. It is the logical and consequent step for our customers towards better process monitoring and safety of their production.
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