Metcal - Profiling for successful BGA/CSP Rework

Our thanks to Metcal for allowing us to reprint the following.


This paper discusses how to successfully profile a printed circuit board when reworking Ball Grid Array and Chip Scale Packages.


Board Profiling for Production vs. Rework


If we consider profiling a PCB for reflow in production, we are generally concerned with achieving an even Delta T (ΔT) across all of the joints being soldered. This is accomplished by evenly heating both the top and bottomside of the whole board, generally through a multi-zoned convection reflow oven. The capacity of the oven would be appropriate to the size or thermal mass of the PCB and the production throughput requirements.


When we consider reworking an array package such as a BGA or CSP, it is recognized that process control is of paramount importance to achieve a successful result. The normal way to approach the rework is to try and emulate the production reflow profile for the individual component being removed or replaced. The nature of the operation dictates that we only wish to reflow the component being reworked and therefore we have to heat a specific area of the circuit board. Selective heating of an area of board can inadvertently cause process failure even when all the parameters appear to be correct. This note discusses the influential factors and the potential pitfalls to be avoided


Process Yield Problems


In some cases, particularly on larger PCB assemblies, failure may still occur, giving low yields when all of the conditions for rework are apparently correct. The requirements for a solder profile are generally well understood. They are normally dependent on the materials being used (solder paste or flux) and would typically be as follows.


On any reflow soldering system that heats both sides of the PCB assembly, the required solder joint temperature is a function of how much heat is applied to both the top and bottom side. It is quite possible to achieve the same solder joint temperature with...

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