Reflow soldering unevenness factor

The primary factor affecting the uneven heating of reflow soldering is that the SMT reflow soldering technology constitutes the uneven heating of the components: the difference between the heat capacity or the heat receiving capacity of the reflow soldering element, the marginal influence of the conveyor belt or the heater, and the load of the reflow soldering product. aspect.

1. In the reflow soldering machine, if the conveyor belt recirculates the transmission product in the week, it will also become a heat dissipation system. In addition, the margin of the heating part is not compatible with the central heat dissipation condition, and the marginal temperature is low, and the temperature in the furnace is removed. The demand is not the same, the temperature of the uniform surface is also different.   

2. The impact of product loading imbalance. The adjustment of the temperature profile of reflow soldering requires consideration of the superior repeatability of the no-load, load and unloading factors. The load factor is defined as: LF = L / (L + S); L = the length of the assembled substrate, and S = the interval of the assembled substrate.     

Reflow soldering technology requires reproducible results, and the harder the load factor, the harder it is. The maximum load factor of the reflow oven is limited to 0.5 to 0.9. This depends on the product (component welding density, non-substrate) and the type of reflow furnace. In order to obtain superior welding results and repeatability, theoretical experience is of the utmost importance.     

Analysis of the cause of welding defects associated with uneven heating of reflow soldering     

1, reflow soldering bridge    

The solder sag will also occur during the welding and heating process. This situation is present in both preheating and main heating. When the preheating temperature is within the limits of tens to 100, the solvent as one of the components in the welding evaluation will decrease the viscosity. And if it flows out, if the tendency of the outflow is severe, the solder particles will be extruded out of the gold-containing particles outside the soldering zone, and if it cannot be returned to the soldering zone during the melting, it will also constitute a solder ball.     

In addition to the above elements, whether the SMD component terminal electrode can be flattened, whether the wiring of the circuit route board and the soldering zone can be standardized, the selection of the solder resist coating method and the coating precision thereof may constitute a bridge.     

2, reflow soldering monument (Manhattan phenomenon)     

The slanting of the chip component in the case of rapid heating is caused by the temperature difference between the two ends of the rapid heating element, the solder on one side of the electrode point is completely melted to obtain superior wetting, and the solder on the other side is completely melted to cause poor wetting. How to promote the erection of the components. Therefore, when heating, it is necessary to think from the viewpoint of the time element, so that the degree of heating tends to constitute a balanced temperature distribution, and avoid the onset of rapid heat.