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PZTA42T1 Folha de dados(PDF) 4 Page - Motorola, Inc |
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PZTA42T1 Folha de dados(HTML) 4 Page - Motorola, Inc |
4 / 6 page PZTA42T1 4 Motorola Small–Signal Transistors, FETs and Diodes Device Data SOLDER STENCIL GUIDELINES Prior to placing surface mount components onto a printed circuit board, solder paste must be applied to the pads. A solder stencil is required to screen the optimum amount of solder paste onto the footprint. The stencil is made of brass or stainless steel with a typical thickness of 0.008 inches. The stencil opening size for the SOT-223 package should be the same as the pad size on the printed circuit board, i.e., a 1:1 registration. SOLDERING PRECAUTIONS The melting temperature of solder is higher than the rated temperature of the device. When the entire device is heated to a high temperature, failure to complete soldering within a short time could result in device failure. Therefore, the following items should always be observed in order to minimize the thermal stress to which the devices are subjected. • Always preheat the device. • The delta temperature between the preheat and soldering should be 100 °C or less.* • When preheating and soldering, the temperature of the leads and the case must not exceed the maximum temperature ratings as shown on the data sheet. When using infrared heating with the reflow soldering method, the difference should be a maximum of 10 °C. • The soldering temperature and time should not exceed 260 °C for more than 10 seconds. • When shifting from preheating to soldering, the maximum temperature gradient should be 5 °C or less. • After soldering has been completed, the device should be allowed to cool naturally for at least three minutes. Gradual cooling should be used as the use of forced cooling will increase the temperature gradient and result in latent failure due to mechanical stress. • Mechanical stress or shock should not be applied during cooling * Soldering a device without preheating can cause excessive thermal shock and stress which can result in damage to the device. TYPICAL SOLDER HEATING PROFILE For any given circuit board, there will be a group of control settings that will give the desired heat pattern. The operator must set temperatures for several heating zones, and a figure for belt speed. Taken together, these control settings make up a heating “profile” for that particular circuit board. On machines controlled by a computer, the computer remembers these profiles from one operating session to the next. Figure 2 shows a typical heating profile for use when soldering a surface mount device to a printed circuit board. This profile will vary among soldering systems but it is a good starting point. Factors that can affect the profile include the type of soldering system in use, density and types of components on the board, type of solder used, and the type of board or substrate material being used. This profile shows temperature versus time. The line on the graph shows the actual temperature that might be experienced on the surface of a test board at or near a central solder joint. The two profiles are based on a high density and a low density board. The Vitronics SMD310 convection/infrared reflow soldering system was used to generate this profile. The type of solder used was 62/36/2 Tin Lead Silver with a melting point between 177 –189 °C. When this type of furnace is used for solder reflow work, the circuit boards and solder joints tend to heat first. The components on the board are then heated by conduction. The circuit board, because it has a large surface area, absorbs the thermal energy more efficiently, then distributes this energy to the components. Because of this effect, the main body of a component may be up to 30 degrees cooler than the adjacent solder joints. STEP 1 PREHEAT ZONE 1 “RAMP” STEP 2 VENT “SOAK” STEP 3 HEATING ZONES 2 & 5 “RAMP” STEP 4 HEATING ZONES 3 & 6 “SOAK” STEP 5 HEATING ZONES 4 & 7 “SPIKE” STEP 6 VENT STEP 7 COOLING 200 °C 150 °C 100 °C 50 °C TIME (3 TO 7 MINUTES TOTAL) TMAX SOLDER IS LIQUID FOR 40 TO 80 SECONDS (DEPENDING ON MASS OF ASSEMBLY) 205 ° TO 219 °C PEAK AT SOLDER JOINT DESIRED CURVE FOR LOW MASS ASSEMBLIES DESIRED CURVE FOR HIGH MASS ASSEMBLIES 100 °C 150 °C 160 °C 170 °C 140 °C Figure 2. Typical Solder Heating Profile |
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