SMT/THR PCB-COMPONENTS

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3 THE SMT PROCESS FOR MODULES WITH THR COMPONENTS 3.3 Paste printings Preparation of the PCB, i.e. applying solder paste on the soldering pads, is one of the most important steps in the SMT production process. The quality achieved here has a crucial influence on the quality of all further process steps. The soldering result in the subsequent placement and reflow process is determined by the necessary paste volume and thus the degree of solder paste filling during paste application. For optimum filling of the THR holes within the usual process tolerances, printing of the solder paste in two different stencil printing processes has asserted itself, namely the open and closed stencil printing processes. In the open stencil printing process, a metal or plastic squeegee pushes the roll of solder paste over the stencil. The drawback of this process is that both the pressure and the printing speed can only be varied within a very limited range. Open stencil printing process In closed printing systems such as "ProFlow", however, a paste deposit with a variably adjustable pressure travels over the stencil. The advantage: better monitoring of the pressure and printing speed permits paste printing by High- Speed Fine-Pitch Technology as well as controlled filling of the THR hole. Various stencil variants can be used depending on the chosen technology, namely the low-cost standard metal stencils or even expensive "exotic" variants such as stepped or double printing stencils. For an optimum degree of solder paste filling, the following process parameters must be taken into account in the paste printing process: • stencil thickness: generally 120 to 200 µm • optimum coordination of the solder paste grain size (20-40 µm) and volume percentage of the flux (mostly 50%) • coordination of squeegee speed and pressure: a higher degree of filling with open paste systems is achieved by a lower speed or a flatter angle (e.g. 45° instead of 60°) Pin header (open and closed ended): 2 to 8 poles 9 to 24 poles Pin header (flanged LF): 2 to 24 poles recommended inside diameter d I = 1.4 +0.1 mm d I = 1.5 +0.1 mm finished hole *1): paste volume VP [mm 3 ] / filling level fp [%] after print minimum solder joint shape 2.4 mm 3 / 70 % 3.1 mm 3 / 85 % optimum solder joint shape 2.9 mm 3 / 90 % 3.5 mm 3 / 100 % Example of a recommended paste volume or degree of filling for SL-SMT pin headers accord. to data sheet 10 APPLICATION INFORMATION
Various requirements for the leaded components can now be derived from these parameters: • as the housing of the leaded component must not touch the solder paste, its stand-off height should be at least 0.3 mm • the pin cross-section geometry, the pin tip and its length must be optimised to minimise the paste volume needed If the degree of solder paste filling is too low, then it is advisable to overprint to achieve the necessary paste volume. Overprinting means that the stencil hole diameter is designed to be larger than the soldering eyelet diameter and the paste is printed over an area larger than the soldering eyelet, and out onto the PCB. This can, of course, cause soiling of the underside of the stencil, which can be removed by a shorter cleaning cycle of the stencil. Overprinting can lead to soiling of the underside of the stencil Solutions and recommendations from Weidmüller To realise the lowest possible paste volume in stencil printing, the SL-SMT pin headers from Weidmüller feature short octagonal pins with chamfered pin ends. Above all thanks to the pin length of only 1.5 mm for standard pc boards with a thickness of 1.6 mm, a filling degree of 90 % only is adequate for an optimal solder joint shape. Therefore in most cases normal single-layer stencils for fine-pitch technology and the standard process parameters for the squeegee speed and pressure can be used during paste printing. Further advantage: in the placement process, the paste is forced out of the placement hole to a lesser extent. APPLICATION INFORMATION 11
Page 1 and 2: SURFACE-MOUNT-TECHNOLOGY PCB-COMPON
Page 3 and 4: TABLE OF CONTENTS TABLE OF CONTENTS
Page 5 and 6: 2 THR THE REVOLUTION IN PCB ASSEMBL
Page 7 and 8: 3.2.1 Defining the diameters of pla
Page 9: Additional design factors : Stencil
Page 13 and 14: % Changes in length because of wate
Page 15 and 16: • Thermal coefficient of expansio
Page 17 and 18: • circumferential wetting of the
Page 19 and 20: SL-SMT are innovative! SL-SMT featu

SMT/THR PCB-COMPONENTS

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