Because some lead-free components need to be used on the original tin-lead circuit board, backward compatibility issues have arisen.
Regarding the backward compatibility issue, some components only have lead-free surface treatment. For component suppliers, it is not cost-effective to provide both tin-lead and lead-free versions of the same component. Lead-containing components with lead-free surface treatment can be used without problems. However, using lead-free BGAs on an original tin-lead circuit board creates a problem. Since all other components are tin-lead components, if a tin-lead soldering temperature profile with a maximum peak temperature of 220°C is used, the lead-free BGA solder balls will only partially melt, or reflow soldering may not be achieved at all, leading to a series of solder joint reliability issues. So, which reflow soldering temperature profile should we actually use? Here are two options:
The first approach is to use a standard tin-lead reflow soldering temperature profile. Except for the lead-free BGAs, the peak reflow temperature for all components is between 210°C and 220°C. Therefore, lead-free BGAs should not be soldered together with other tin-lead components. After the tin-lead components have completed reflow soldering, selective soldering is used, that is, a selective laser soldering system is used to place and solder all lead-free BGAs. The selective laser soldering system only places and solders the lead-free BGAs and does not affect the surrounding tin-lead components that have already been soldered in the convection reflow oven.
The second approach is that if there is no tin-lead soldering temperature profile, and you want to solder all tin-lead components and some lead-free BGAs in the same oven, then the reflow peak temperature must not damage the tin-lead components, but must be sufficient to reflow the lead-free BGAs. Don't forget that since most components on the circuit board are tin-lead components, you need to use tin-lead solder paste. Therefore, a peak temperature between 210°C and 220°C is suitable for tin-lead components, but it is insufficient for lead-free BGAs with a melting point between 217°C and 221°C. If the peak temperature is 226°C to 228°C, with a time above liquidus (TAL) of 45 to 60 seconds, this is sufficient to reflow the lead-free BGAs without damaging all the tin-lead components on the same circuit board.
If the reflow temperature range of 226°C to 228°C is too narrow to complete the soldering of backward-compatible tin-lead components and lead-free BGAs, consider using selective laser soldering, or look for suppliers that provide BGAs with tin-lead solder balls. When developing any temperature profile, it is important to use the correct thermocouples. We need K-type thermocouples with a 36 AWG wire attached. If the thermocouple wire is thicker, it will absorb too much heat. Never use thermal tape, because it will loosen during the reflow process and measure the temperature of the air in the oven, not the temperature of the solder joint. In any case, high-temperature solder or thermally conductive adhesive must be used to attach the thermocouple to the solder joint.
For BGAs, start from the bottom of the circuit board, drill holes in the inner and outer ring BGA pads, and push the thermocouple to the highest point near the surface to measure the temperature of the BGA solder balls. The temperature difference between the inner and outer ring BGA solder balls must be within 2°C. Place four to six thermocouples at different component locations to describe the lowest to highest heat capacity areas, with at least two thermocouples used for BGAs.
? ?There is a misconception that a reflow temperature profile from a convection reflow oven is suitable for all circuit boards, and therefore, it is not necessary to develop a specific reflow temperature profile for each circuit board. This is incorrect. Because each circuit board has a different heat capacity, and each circuit board has a different assembly pattern. For the same double-sided circuit board, each side may require a different reflow temperature profile depending on the component layout and copper foil distribution on each side. There is also a misconception that if you want to change the reflow temperature profile, you can do so by changing the conveyor belt speed. Simply changing the conveyor belt speed is easy, but it is not the correct method, because it will change the temperature of the circuit board in each temperature zone. Complete hardware and software packages are now available to simplify the development of reflow temperature profiles.
? ?Once the expected reflow temperature profile is obtained, production can proceed with boards that have been printed with solder paste and populated with components; after reflow soldering, inspect the quality of the solder joints. Random problems that occur only at a specific location on the circuit board may be related to soldering; problems that consistently occur at a specific location may be due to uneven heating and related to the temperature profile. Problems that occur throughout may also be related to solder paste quality and pad pattern design.
? ?When the reflow temperature profile gives ideal results (assuming that design and other material variables have been optimized), then this temperature profile should be locked in and should not be changed.
