Reflow soldering stage in the BGA assembly process

On the production line of the SMT assembly plant, reflow soldering is a  difficult process in the BGA assembly process. Setting process  parameters and obtaining a suitable temperature profile are very  important for good BGA soldering. Due to the different packaging forms  of BGA, the thermal resistance of CBGA is larger than that of PBGA, so  to reach the same temperature, CBGA requires a higher temperature  setting and longer preheating time than PBGA. For tin-lead solder paste  and lead-free solder paste, the temperature setting and heating time are  significantly different.

reflow soldering machine

1. Pre-heating stage

The main purpose of preheating is to heat the PCB and its components  uniformly, and at the same time have a baking effect on the PCB and  components, remove the moisture in it, and evaporate the appropriate  amount of flux in the solder paste. The heating rate in the preheating  stage cannot be too fast to prevent the PCB from being heated too fast  and causing greater deformation. Generally, the heating rate is  controlled at 3℃/s, and the preheating time is between 60-90s.

2. Activation stage

The main purpose of this stage is to activate the flux in the solder  paste, remove the oxides on the surface of the pad and the surface of  the solder paste alloy, to achieve a clean metal surface and prepare for  the solder paste reflow process. At the same time, the excessive flux  in the solder paste is evaporated and the PCB is preheated to prevent  the PCB from deforming due to excessive heating during the reflow  process. For tin-lead soldering, the temperature at this stage should be  kept at 60-120S at 150-180℃; for lead-free soldering, the temperature  at this stage should be kept at 60-180S at 160-200℃, so that the flux  can fully play its role. The temperature rise rate in the activation  stage is generally controlled at 0.3 plus. 5°C/s.

3. Reflow stage

At this stage, the temperature of the solder joint has risen above the  melting point of the solder paste, and the solder paste is in a molten  state. The main purpose of the reflow stage is to wet the solder pads  and the pins of the components to achieve good soldering requirements.  For PBGA, the solder balls are Sn63Pb37, Sn62Pb36Ag2, and  Sn96.5Ag3.0Cu0.5 alloys. During the reflow process, the solder balls and  solder paste are melted and mixed to form solder joints; for CBGA, the  solder balls are Snl0Pb90 high-temperature alloys. The solder ball does  not melt during the reflow process, the solder paste melts and wets the  pad and the high-temperature solder ball to form a solder joint.  Therefore, it takes a suitable time to ensure that the molten solder  paste can well wet the pads and solder balls. Too short a time may cause  poor wetting to form a pseudo solder, and too long may cause a thick  formation between the solder and the pad, a layer of intermetallic  compounds Cu6sn5 and Cu3Sn, due to its brittleness, is easy to form  cracks and cause solder joint failure. Especially for lead-free  electronic assembly, due to the high content of element Sn in lead-free  solder alloying, it is easier to form thicker intermetallic compounds at  high temperature and cause the failure of solder joints. For SnPb  soldering, the time above the melting point of 183℃ is generally  controlled in 60-90s, where the peak temperature is within the range of  210-225°C and the time is controlled within 10-20s; for lead-free  soldering, it is generally required to control the time above the  melting point of 217-219°C within 60-120s, and the peak temperature  230-235℃ is better to control the time in the range of 20-40s.

reflow temperature profile

4. Cooling stage

After the solder paste is reflowed, the flux is completely consumed,  forming a molten metal solder joint. The main purpose of the cooling  stage is to refine the crystal grains while the solder joints are  solidified, inhibit the growth of intermetallic compounds, and improve  the strength of the solder joints. However, too fast cooling rate will  cause PCB deformation and thermal cracking of electronic components,  especially for components with large heat absorption such as BGA, too  fast cooling rate will easily cause damage to the internal package,  leading to BGA failure. Generally, the cooling rate is controlled within  1-3℃/s.