Influence of Nitrogen and Vapour Phase Soldering on the Formation of Tombstone Effect_Fitech Solder Paste
Influence of Nitrogen and Vapour Phase Soldering on the Formation of Tombstone Effect
In the surface assembly process of electronic products, especially in the high-volume reflow soldering process, the tombstone effect of passive chip components to the PCBA assembly soldering adds a lot of trouble. With the SMC / CMD micro-miniaturisation, reflow soldering of these chip components will appear "upright", this phenomenon is also known as the "Manhattan" effect.
Figure 1. Tombstone phenomenon
This phenomenon occurs mainly in small chip components, they are soldered on the opposite poles of the surface mount pads, in the soldering process, the components stand up vertically, as shown in the figure. Sometimes it is partially upright, and sometimes the component is completely upright on a pad, like a tombstone in a cemetery.
With the miniaturisation and lightweighting of chip components and the application of high-temperature lead-free solders, the effect of "tombstone" defects has become more significant. Tombstoning effects tend to occur in gas-phase reflow and nitrogen reflow systems, and even when using new batches of components and PCBs.
One of the causes of the "tombstone" effect is the difference in the initial wetting force of the solder on the two ends of the passive component, which results from the difference in the wetting force of the surfaces of the two solder ends and the surface tension of the solder. If one solder end is reflowed and wetted more quickly than the other, the forces acting on that end to form a solder joint may cause the other solder end of the component to lift, creating the tombstone phenomenon.
The wetting mechanism consists of three important parameters: initial wetting time, wetting force and complete wetting time. Complete wetting occurs when the maximum force is applied to the solder joint and the component, so the difference in complete wetting time is directly related to the generation of tombstone defects.
The effect of nitrogen and gas phase soldering on tombstone effects is significant. During reflow welding, nitrogen prevents surface re-oxidation and accelerates the onset of initial wetting; the rate of temperature rise in gas phase welding should be controlled, and like the nitrogen environment, the gas phase of gas phase welding prevents surface re-oxidation. Under both process conditions, the component to be welded has a much reduced oxidation of the metallised surface as it enters the reflow zone, allowing faster wetting. If the reflow soldering temperature rises too quickly at this point, or if the direction, speed and temperature of the airflow are not uniform, then the SMD component is more likely to be pulled up, creating a tombstone effect.
The faster initial wetting does not provide enough time to reduce the ΔTS, and the additional delay in the initial wetting time to reduce the ΔTS is exactly what is necessary to reduce the occurrence of tombstone defects. Therefore, while we see the obvious benefits of nitrogen and gas phase welding, we need to take extra precautions to minimise the occurrence of tombstoning.
1. Adjust the reflow temperature curve, increase the preheating temperature and preheating time to reduce the temperature difference between the two ends of the component;
2. The use of nitrogen protection reflow soldering, to control the amount of residual oxygen in the nitrogen environment, preferably 500ppm;
3. Under the premise of ensuring the strength of the solder joint, the pad size should be as small as possible, because after the pad size is reduced, the amount of solder paste applied is reduced accordingly, and the surface tension of the solder paste melting is also reduced. The heat capacity of the entire pad is reduced, the probability of melting solder paste on both pads at the same time greatly increased.
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