Study of solder jet bumping process using high-speed digital camera

With the development of solder jet technology in the electronic packaging industry, the bumping process of a molten metal droplet, which determines the shape of the solder bump and is crucial for the performance of the device, has attracted great interests. The solder bumping process of a single molten micro-droplet by a solder jet was recorded using a high-speed digital camera with a frame rate of 100,000 frames per second. It was found that the surface ripples on the solder bump was caused by the interaction of the fluid flow and the heat transfer/solidification processes in the bumping process of a micro-droplet. A droplet was observed to rebound on a copper pad coated with a layer of organic solderability preservatives, which was suspected to decrease the interfacial heat transfer coefficient between the droplet and the pad lower than a minimal value, 4.07×104 W/m2 K, making the recoiling droplet rebound away.