The application of furnace temperature tester in reflow soldering

The variation of the temperature curve inside the furnace is usually related to the quality and effectiveness of the processed product, so the furnace temperature tester has certain technical requirements for the reflux temperature curve. Generally speaking, the reflux temperature curve can be divided into three stages: preheating stage, reflux stage, and cooling stage.

1、 Preheating stage

The preheating temperature is generally set within the range of 80-160 ℃ to gradually increase in temperature (optimal curve); For the traditional curve constant temperature zone between 140 and 160 ℃, it should be noted that if the temperature is high, the oxidation rate will accelerate significantly (it will increase linearly in the high temperature zone. At a preheating temperature of around 150 ℃, the oxidation rate is several times higher than at room temperature, and the relationship between copper plate temperature and oxidation rate is shown in the attached figure). If the preheating temperature is too low, the activation of the flux will not be sufficient.

The preheating time depends on the component with the highest heat capacity on the PCB board, PCB area, PCB thickness, and the performance of the solder paste used. Generally, the preheating period between 80-160 ℃ is 60-120 ° C, which effectively removes volatile solvents in the solder paste, reduces thermal shock to the components, fully activates the flux, and reduces the temperature difference.

Preheating temperature rise rate: In terms of the heating stage, a slow rise rate in the temperature range between room temperature and melting point temperature is expected to reduce most defects. For the optimal curve, a slow rise rate of 0.5-1 ℃/sec is recommended, while for traditional curves, it is better to heat up below 3-4 ℃/sec.

Preheating is a heating behavior carried out to activate the tin water and avoid rapid high-temperature heating during tin immersion, which may cause parts to fail to meet the intended purpose.

2、 Reflux stage

The peak temperature of the reflow curve is usually determined by the melting point temperature of the solder, the heat resistance temperature of the assembled substrate and components. The minimum peak temperature is generally around 30 ℃ above the melting point of solder (for current Sn63 Pb solder with a melting point of 183 ℃, the minimum peak temperature is about 210 ℃). If the peak temperature is too low, it is easy to produce joints and insufficient wetting, resulting in insufficient melting and the formation of half fields. Generally, the highest temperature is about 235 ℃. If it is too high, the epoxy resin substrate and plastic parts are prone to coking and delamination. In addition, excessive co bonding metal compounds will form, leading to brittle welding points (welding strength impact). The time is generally set between 45 and 90 seconds, and this time limit requires the use of a fast temperature rise rate, which rapidly rises from the melting point temperature to the peak temperature, while considering the thermal stress factor that the component bears. The rise rate must be between 2.5 and 3.5 ℃/sec, and the maximum change rate cannot exceed 4 ℃/sec.

3、 Cooling stage

A slow cooling rate above the melting point temperature of the solder will lead to the production of excessive eutectic metal compounds, as well as the occurrence of large grain structures at the solder joint, resulting in a decrease in the strength of the solder joint. This phenomenon generally occurs within the temperature range of the melting point temperature and slightly below the melting point temperature.

Rapid cooling of the tester will result in a high temperature gradient between the component and the substrate, leading to thermal expansion mismatch, splitting of solder joints and pads, and deformation of the substrate. Generally, the maximum allowable cooling rate is determined by the tolerance of the component to thermal shock. Taking into account the above factors, the cooling rate in the cooling zone is generally around 4 ℃/S, and cooling to 75 ℃ is sufficient.