SMT/THR PCB-COMPONENTS

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3 THE SMT PROCESS FOR MODULES WITH THR COMPONENTS 3.6 Quality control Quality control concludes the SMT production process. The aim of this process step is to enable swift and simple assessment, by suitable means, of the quality of the soldered joints of THR components with short or long pins. The same rules apply to quality control of the THR technology as to reflow or wavesoldered THT components. Inspection method: Quality control in parallel with the process can be realised by means of different processes. At the moment, mostly optical or X-ray inspection is used. The optical test evaluates the shape, reflection and colour of soldered joints. In the case of X-ray inspection, so-called radiographic evaluation is run by means of an automated X-ray microscope. In addition the inspection processes during the production process, an important inspection step consists of destruction tests. Carried out on samples, they serve to check measurable quantities: Cross-section of the THR-solder joint with short pin • analysis of the degree of filling by means of a cross-section through the soldered joint • mechanical inspection of the force needed to pull the pins out of the through plated holes a Standards: Various standards have come into force in the past years to enable objective quality control. Besides other acceptance test criteria for the production quality of electronic modules, the assessment criteria based on the IPC-A-610B have been acknowledged the world over. According to IPC-A-610B, the quality requirements for THR soldered joints are subdivided into three categories. Generally the requirements of category 3 are decisive for high-power electronics in industrial applications. The following values have been defined for the total number of five assessment criteria: • circumferential wetting of the primary side (i.e. the component side from the component's point of view), the terminal and the sleeve must not fall below 270° • vertical solder filling must be at least 75% Optical test of soldered joints below the housing body. When done manually, a magnifying glass or a microscope is used and, in the automated process, a computercontrolled camera performs computerassisted image evaluation. 75 % Dimensioning the filling height for through-hole soldered joints in accordance with IPC-A-610B 16 APPLICATION INFORMATION
• circumferential wetting of the secondary side (i.e. underside from the point of view of the component) must not fall below 330° • solder wetting of the original land face (i.e. residual ring in the case of THR soldered joints) on the primary side is defined as 0% • solder wetting of the original land face on the secondary side must be at least 75% With regard to quality control, these standards result in several requirements for leaded components. It is necessary to take into account the fact the two different designs are normally used for the THR process: • shorter pins in comparison with the PCBs thickness • longer pins in comparison with the PCBs thickness (approx. 1.0 – 1.5 mm overhang) The following requirements apply in the case of short pins: • the soldered joints on the primary side of the PCB must be visible • in the case of components with pins under the insulator, the height of the stand-off or the product design must be adequately dimensioned for visual inspection These requirements apply to long pins: • the soldered joints on the secondary side must be visible • by comparison, soldered joints on the primary side are not necessary In the case of destructive inspections in conjunction with THR components, demands are also placed on the PCB. The force needed to pull the pins out of the soldered joint is checked to ensure the quality of PCB production: Required pull-out force: - for short pins: approx. > 150 N - for long pins with solder meniscus on the primary and secondary sides: approx. > 220 N Solutions and recommendations from Weidmüller Short pin pulled out with pull-out force of >150N Short pins with a solder meniscus only on the primary side attains a comparably high stability which is otherwise only possible when using the conventional THT. Hence THR components - like Weidmüllers SL-SMT - fitted with short THR pins have a considerable advantage over real surface-mount connections. The special design of the SL-SMT pin headers from Weidmüller permits visual inspection of a PCBs primary side, even in the case of variants with an 180° outgoing direction, thus creating the prerequisite for quality control in parallel with the process. However it is important that the control side of the components is not concealed by other high elements. Weidmüller recommends therefore, during the layout of the PCB, to take into consideration the placement possibilities of the components on the primary or quality inspection side. For comparison: in the case of surface connections with SMD components, the required pull-out force amounts only to about 15 – 20 N. APPLICATION INFORMATION 17
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 and 10: Additional design factors : Stencil
Page 11 and 12: Various requirements for the leaded
Page 13 and 14: % Changes in length because of wate
Page 15: • Thermal coefficient of expansio
Page 19 and 20: SL-SMT are innovative! SL-SMT featu

SMT/THR PCB-COMPONENTS

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