Development of a process for the production of fluxless solder spheres for ball grid array electronics packaging

Thesis (M.Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1999.Includes bibliographical references (leaves 84-86).The work presented here focuses on the use of a new process for the production of fluxless solder spheres for BGA (Ball Grid Array) microelectronics packaging technology. The Uniform Droplet Spray (UDS) process produces solder spheres by controlling the breakup of a continuous laminar jet into uniform droplets. which are then rapidly solidified in a liquid bath or an inert gas. This work discusses the production of highly size-accurate solder micro spheres (75-1000 [mu]m in diameter) by the UDS process. To achieve the sphere size distribution requirements an on-line droplet size control system was developed. Dropiet size control is accomplished by performing a real-time measurement of the droplet size and compensating for the difference between actual and target sizes by adjusting the break up frequency. The control system is effective in controlling sphere size, enabling the UDS process to accurately determine and control solder sphere size within± 2.5% of target size. Current BGA assembly processes require that the oxide films. typically up to I 00 A thick, on the surface of both the solder sphere and the substrate. be removed. or otherwise chemically altered. to permit melting. wetting. and spreading of the solder and good bonding characteristics. The common technique for cleaning the surfaces while soldering is to use a liquid solder flux. However. the flux residues have to be cleaned using chlorofluorocarbon (CFC) solvents. Because of environmental and worker-safety concerns. CFC will be eliminated in the next few years. Thus the need to develop a process for producing fluxless solder spheres for BGAs is a critical issue. This work also presents the coupling of a plasma-assisted treatment and the UDS process for the production of fluxless solder spheres by transforming the chemical composition of the spheres' surface. The solder spheres are treated in fluorine-containing plasma to convert the metallic surfaces into oxy-fluorine layers. These layers protect the spheres from further oxidation and act as flux during the reflow process. Experimental evaluation of the solderability performance of the fluxless solder spheres indicates that the plasma-assisted oxide conversion process is effective in replacing flux. enhancing operator safety, reducing process time and production cost. and minimizing contamination of the work environment.by Juan Carlos Rocha Alvarez.M.Eng