![]() Oxygenated reflow environments are often not compatible with the flux in solder paste, leading to non-solderable oxidized surfaces and wettability issues. Macro View of LED component mount by reflow solderingĪ reflow environment with nitrogen is often recommended or required by assembly specifications to reduce surface oxidation. At this point, the solder powder spheres combine with the board and components. Too much time at this temperature can cause damage to the components or can lead to brittle solder joints.Īt the melting point, the flux within the solder paste reduces surface tension at the juncture point of the metals to achieve metallurgical bonding. This time is typically 30 seconds to 1 minute. This time is critical to ensure appropriate “wetting” materializes between the components and board. “Time above liquidus” (TAL) is also established - the length the solder is held above its melting point. ![]() This maximum temperature is a delicate point that needs to be quite precise - it can not be too hot (to avoid thermal damage to board and/or components) or too cool (to avoid poor coalescence / solder joints). This maximum should be below the component that has the lowest maximum temperature allowance, but above the reflow melting point of the solder paste used. It is critical that the maximum temperature is established properly during the reflow profile definition. The solder flux activates and the powder melts, forming the joint, while the flux evaporates away. In this step, the solder paste is melted to fuse the solder joint. The reflow step brings the oven, board, components, and solder to the maximum temperature during the entire process. Likewise, it’s critical the soak phase is done at the perfect temperature or the flux (flowing agent in the solder paste) can suffer from defects like graping and/or head-in-pillow (HIP or HNP). It’s important to note that the initial heating step is critical prior to the soak step to ensure the heating rate is sufficiently gradual. The thermal soak step lasts just 1 to 2 minutes and will get all the components to the exact required temperature for reflow in step 3. Image Source: National Physics Laboratory Soldering Defects Database Solder beading, pictured above, is just one of several issues that can arise from excessive heating & poor SMT reflow profile. Components can also be damaged if heating guidelines are not properly followed during these steps. ![]() Solder beading will fail minimum gap requirements, affecting the board’s electrical reliability. Heating the solder too quickly can cause splattering, leading to solder balls/beads. The heating rate during this step is defined by the solder paste’s datasheet and guidelines but usually is 2✬/second or less. The initial heating step is to ensure the assembly and solder reach a specified temperature at a specific rate, also known as ramp rate. Your browser does not support HTML video. Reflow Step One: Initial Heating / Preheat The board then goes into the reflow oven to form the solder joints and bind the components to the board. For double-sided boards, the board is flipped and the process is repeated. A pick-and-place machine then places the components on the board. Preceding the reflow soldering process, a stencil is created and used to apply the solder paste to the contact pads on the PCB.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |