PCB + ASSEMBLY Source: Indium For the water-soluble solder paste, the voiding is affected by different profiles. However, with electroformed stencils, one array prints encountered bridging. The amount of bridging increased from column 1 (50 μm gap) to column 5 (150 μm gap). The increased transfer efficiency of the electroformed stencil is most likely the cause for this distinct difference in print performance. The other array with reduced apertures did not give any bridging. Comparison of solder pastes On the other array, the aspect ratio ranges from 0.40 to 0.45 only for a 50 μm thick stencil. However, a consistent print is still achievable for these pads. Comparing the deposit volume of the three pastes with different aspect ratios shows: paste C ranks the best among the three pastes; paste A ranks second; and paste B is third. Paste B has a tighter distribution compared to paste A and paste C; however, it has many more insufficient compared to the other two pastes. The rheology plays a big role in printing ultra-fine apertures, and good printability is achievable with both water-soluble and noclean flux chemistries. The 35 μm thickness stencil gave a higher aspect ratio, hence it was believed that the thinner stencil could yield better printings over the 50 μm thick stencil. The aspect ratio improved from a range of 0.40 – 0.45 to 0.57 – 0.64. The improvement was obvious by using paste sample C. The printing results of T6 and T7 pastes show that the standard deviation of T7 prints is smaller, so it has a lower variation. However, it was also observed that more bridges were seen. Process capability Cp, Cpk, and Ppk were calculated using Minitab software. The reference specification limit used was 40 % to 150 % to compare the paste performance. Both vacuum support and carrier pallet are capable setups, as the table reveals; however, vacuum support has a greater consistency as the Cpk and Ppk values are larger. The process capability of all pastes can be seen in the table, using various pad sizes and stencil thicknesses. The combinations of pad size and stencil thickness were represented as their respective aspect ratio. Reflow and test vehicle Reflow testing was done on one of the water-washable pastes that is being used in a high volume SiP application to study the void performance with different reflow profiles in air. The Practical Components test board was used, with pad and stencil apertures both of 180 μm circles. The thickness of the stencil was 75 μm. Four different profiles were tested as shown in the diagrams. The actual profiles were recorded. Results and conclusions The results are summarized in table and figure. For this particular water-soluble solder paste, the voiding is affected by different profiles. The long profile with soak zone shows a lower void %. The relationship between the void performance and reflow profile should be adequately characterized for each solder paste, as different solder pastes may work best with different temperature profiles. In order to achieve consistently good fine-feature printing performance for SiP applications, solder paste attributes, i.e., powder size, flux system, rheology, slump behavior, and stencil life, are important and need to be taken into consideration. Solder paste with suitable rheology, mixed with the correct powder size and flux, should be evaluated and selected accordingly. The suitable stencil technology, design, and thickness, coupled with an appropriate board support system, are also keys for consistently good solder paste transfer efficiency. Reflow profiles need to be properly characterized in order to minimize voiding for different solder pastes. SMTconnect, Booth 5-310 www.indium.com Zusammenfassung In der Automobilelektronik finden sich mit steigender Tendenz SiPs in den vielen Helfersystemen für die Fahrer – insbesondere in Systemen für die Advanced Automated Driver Assistance (ADAS). Doch stellen die SiP-Bausteine deutlich höhere Anforderungen an den Fertigungsprozess der Baugruppen, insbesondere an Lotpaste, Flussmittelchemie und Reflow. Résumé Dans l‘électronique automobile, les SiP se retrouvent de plus en plus dans les nombreux systèmes d‘assistance à la conduite, en particulier dans les systèmes d‘aide à la conduite automatique avancée (ADAS). Cependant, les composants SiP imposent une demande plus importante au processus de fabrication des assemblages, en particulier en ce qui concerne la pâte à souder, la chimie des flux et la refusion. Резюме В автомобильной электронике все чаще применяются корпуса с однорядным расположением выводов (SiP). В частности, это касается множества систем помощи водителю, например Advanced Automated Driver Assistance (ADAS). Однако при производстве к узлам SiP предъявляются более высокие требования. Речь идет о паяльной пасте, флюсе и системах пайки оплавлением. 54 EPP EUROPE April 2019
PCB + ASSEMBLY Importance of drying in printed-circuit board assemblies cleaning No moisture should be left beneath parts There are four steps to successfully clean printed circuit boards assemblies (PCBAs). The wet, scrub and rinse operations are obvious. Wet the board with a pure cleaning fluid. Scrub it using a good quality scrubbing brush. Then rinse it off with more clean fluid. However, the necessary dry step is often overlooked, or in some instances, completely left out of the PCBA cleaning plan. This article outlines the various techniques and tools used when drying electronics circuitry either in a vapor degreaser or at the benchtop. Sheri Pear, Marketing Communications Specialist, MicroCare Corporation PCBA in the basket of vapor degreaser system where it was immersed in the boiling cleaning fluid. There are different ways to dry PCBAs depending on the cleaning process used, including in-line aqueous cleaning, benchtop scrubbing or vapor degreasing. Whether using heat, vapor, wipes or compressed gas, there is a drying method available to ensure assembled circuit boards are not only clean, but also dry for optimum reliability and performance. For reliable performance, circuit boards must not only be clean, they must be dry in order to function properly. Leftover moisture on a PCB surface can lead to a host of problems including corrosion, electrochemical migration, delamination, dendrite growth, pin holes and blow holes and adhesion problems with conformal coatings later in production. Modern PCBAs often have bottom termination components like land grid arrays and QFNs (quad-flat no-leads) with very tight standoff heights. Moisture can get trapped under these components, making drying even more complicated. Batch cleaning & drying with vapor degreasing For bulk or batch drying PCBAs, convection ovens with air knives might be used. However, when using drying ovens, temperature and air flow are major concerns and care should be taken. For example, if the circulating air flow isn’t strong enough or if the temperature isn’t high enough, it might not reach the moisture hidden under low-mounted components. Or if the temperature is too high, Source: Microcare it may cause surface damage to the board itself or the components by baking on any residual contaminants left behind. Vapor degreasing is an affordable and convenient alternative to using drying ovens. The vapor-based procedure is an industrial cleaning process for PCBAs that is quick, consistent and affordable. The operation uses cleaning fluid immersion, combined with vapor rinsing and vapor drying, to remove all types of contaminates including fluxes, pastes, particulates and residue. EPP EUROPE April 2019 55
