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Let's talk about the setting of lead-free reflow soldering temperature curve

Let's talk about the setting of lead-free reflow soldering temperature curve

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SMT Technical literature
2014/10/22 10:54
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Since some lead-free components were required on the original tin-lead circuit board, there was a problem of backward compatibility.
 
 
For backward compatibility issues, some components are only lead-free surface treated. For component suppliers, it is not cost-effective to provide both tin-lead and lead-free components of the same type. Lead-free components with lead-free finish on the surface are not problematic when used. However, the use of lead-free BGA on an original tin-lead board, the problem is coming. Since all other components are tin-lead components, if a tin-lead soldering temperature curve with a maximum peak temperature of 220 °C is used, the lead-free BGA solder balls are partially melted, or reflow soldering is not possible at all, and a series of solder joints are reliable. Sexual problem. So, which reflow soldering temperature curve should we use? There are two options here:
 
The first method is to use a standard tin-lead reflow profile. With the exception of lead-free BGA, the peak remelting temperature of all components is between 210 ° C and 220 ° C. Therefore, lead-free BGA and other tin-lead components should not be soldered together. After the reflow of the tin-lead components is completed, selective soldering is used, that is, a selective laser welding system is used to place and solder all of the lead-free BGA. The selective laser welding system simply mounts and solders the lead-free BGA without affecting the tin-lead components that have been welded in the convection reflow oven.
 
The second method is that if there is no tin-lead soldering temperature curve and you want to solder all the tin-lead components and some lead-free BGAs in the same soldering furnace, the reflow soldering peak temperature must not damage the tin-lead components, but it is enough. Reflow soldering of lead-free BGA. Don't forget, since most components on the board are tin-lead components, you should use tin-lead solder paste. Therefore, the peak temperature is between 210 ° C and 220 ° C, which is suitable for tin-lead components, but for lead-free BGA with a melting point between 217 ° C and 221 ° C, the temperature is insufficient. If the peak temperature is 226 ° C to 228 ° C, the time above the liquidus (TAL) is 45 to 60 seconds, which is enough to reflow the lead-free BGA without damaging all the tin lead on the same board. element.
 
If the reflow temperature range of 226 ° C to 228 ° C is too narrow, it is difficult to complete the backward compatibility of tin-lead components and lead-free BGA soldering, you can consider selective laser welding, or find suppliers that provide tin-lead solder ball BGA. It is important to develop any temperature profile and use the correct thermocouple. We need a K-type thermocouple with a 36 gauge AWG wire. If the thermocouple wire is thick, it will absorb too much heat. Never use warm tape because they will loosen during reflow and measure the temperature of the air in the furnace, not the temperature of the solder joint. In any case, a thermocouple or a thermally conductive adhesive must be used to attach the thermocouple to the solder joint.
 
For the BGA, drill the BGA pads on the inner and outer rings from the bottom of the board and push the thermocouple to the highest point near the surface to measure the temperature of the BGA solder balls. Inner and outer BGA solder balls must have a temperature difference of 2 °C between each other. Four to six thermocouples are placed at different component locations to describe the lowest to highest heat capacity area, with at least two thermocouples being used for the BGA.
 
   There is a misconception that the reflow profile of a convection reflow oven is suitable for all boards, so there is no need to specify a reflow profile for each board. this is not right. Because each board has different thermal capacities, and each board has a different assembly mode. For the same double-sided board, depending on the layout of the components on each side and the distribution of the copper foil surface, different reflow soldering temperature curves may be required on each side. There is also a misconception that if you want to change the reflow profile, you can change the speed of the conveyor to do it. Simply changing the speed of the conveyor belt is easy, but this is not the correct method because it changes the temperature of the board during each temperature zone. A complete set of hardware and software is now available to simplify the development of reflow soldering temperature profiles.
 
   Once the expected reflow profile is obtained, the circuit in which the solder paste is printed and the components are printed can be produced; after reflow soldering, the quality of the solder joints is detected. The randomness problem that occurs only in a specific location on the board may be related to soldering; the problem that has always occurred in a specific location may be due to uneven heating, which is related to the temperature profile. Problems that may arise from beginning to end may also be related to the quality of the solder paste and the design of the land pattern.
 
   When the reflow profile gives the ideal result (assuming the design and other material variables have been optimized), the temperature profile is determined and cannot be changed.