Why is laser reballing required?
We know why the Hi-Rel electronics industry is not moving to lead free, but instead staying with tin/lead.
While Tin Whiskers is a major factor, there are others, including lead free requiring a higher reflow temperature. This is a particular issue with BGA components, while components like a QFP/TSOP has its terminations at the side, and therefore exposed to the heat of the reflow oven to enable reflow.
A BGA though has its solder terminations below the component, hidden from direct heat of the oven.
So if the Hi-Rel company is using a lower temperature tin/lead process, using lead free BGA’s where the balls are hidden from the heat and require a higher temperature to reflow them, is problematic. There may be not enough heat to reflow the solder balls properly. This is one of the reasons they change the BGA’s from lead free to tin lead balls.
All manufacturers of components specify the amount of reflow cycles their components are specified for, and its generally 3.
The traditional method of reballing involves a reflow cycle. However, if the assembly process is a double reflow, then these BGA’s are at their limit after assembly, or perhaps exceeded it if the lead free ball removal process has induced heat. This is unacceptable risk where long term use is required, in a high reliability process such as aerospace.
Retronix have resolved this issue by creating a laser reballing process that does not involve a reflow cycle. This is coupled with various ball removal methods including solder wave & air knife which offer non-abrasive alternatives to solder braid.
Here’s how the laser works :
The machine uses standard industry solder balls, including lead free, tin/lead & high melting point. It can also place copper cored balls. A dic spins at high speed below the reservoir picking up individual spheres. Each sphere is taken to the nozzle where a flow of Nitrogen blows it through. When the ball hits the pad the Nitrogen sensor senses some back pressure, and knows the ball is on the pad, so the laser is a controlled burst that reflows the ball on the pad perfectly. This is repeated (each ball individually attached) until the BGA has been reballed. Immediately after completion, the BGA can be picked up by hand and you would notice no discernible rise in termperature, hence no reflow cycle. If you do this when a reballed BGA exits a reflow oven, the 200+ degree heat will burn your hand.
Here is the reball process in action –