boards. A batch configuration allows for a higher solvent exposure time by allowing boards to be processed in a parallel manner. Canisters of boards can be loaded into cascading solvent tanks and moved to subsequently cleaner and cleaner tanks (Figure 1). Generally, at least three cascading tanks are used. These consist of clean, rinse 1 and rinse 2. This configuration provides a greater throughput in a smaller amount of space and allows a system to accommodate a wide range of board sizes. Impingement normal to board surfaces is not possible in this configuration. However, pumps can be used to generate turbulence in the solvent tanks. In the batch system, chemical dissolution time can be maximized, but mechanical displacement is limited.
Figure 2. Example of an in-line cleaner.
In-line configuration, on the other hand, consists of spray impingement of every board. Boards travel through the cleaner on a wire mesh belt with spray impingement normal to the plane of the boards (Figure 2). Since boards travel through the machine in a serial manner, the overall system for cleaning and rinsing can be very long. This transport design is mechanically more complex than the batch system, and limits changes in board width. An in-line configuration provides a high degree of mechanical energy, but limits the time of solvents exposure — limiting the time available for dissolution.
Vapor phase solvent blends possess good wetting characteristics and extremely low solvent viscosity and surface tension. This assures flow in all areas beneath flip chip die, allowing dissolution and removal of flux residues. Flux residue on flip chips with fine pitch and low stand-off heights can adversely affect the flow of underfill beneath the die, resulting in voids.
The current cleaning process uses a vapor-phase fluid based on hydrofluorocarbon azeotropic cleaning chemistry (ACC). The solvent components within an azeotropic mixture have thermodynamic fluid properties that allow the ingredients to form a constant-boiling mixture. The composition in the boil, vapor and rinse stages remains relatively constant over time.
The limitation, therefore, of the current ACC cleaning chemistry for flip chips is the incomplete removal of solder flux residues. Several design and assembly factors have been found to directly affect the amount of flux residue remaining after cleaning. For a given flux, the initial amount of flux dispensed for the reflow solder operation, peak reflow temperature, length of time between the reflow assembly and cleaning, and most significantly, the gap height and dimensional size of the flip chips make for a difficult cleaning challenge. Flip chips with organic die passivation layers seem to be more difficult to clean than those with inorganic passivation layers. The board complexity of new designs calls for tighter pitch, higher I/O and lower standoffs.
Nature of Flux Residues
White residue around the periphery of the solder bump is a common concern with cleaning flip chip assemblies. These residues are composed of fluxing byproducts consisting mostly of unreacted soluble organic compounds and a small amount of metal salts of tin that are highly stable in inert (Figure 3). The salts are encapsulated in the remaining flux vehicle. Selecting a cleaning fluid that has the strength to dissolve the vehicle and allow removal of the salts in the initial cleaning process is the key to removal of these residues.
Figure 3. Example of white residue.
The formation of metal salts is caused by the active ingredients in the flux chemistry. The fluxing agents react with the tin and lead oxides on the solder's surface. The breakdown of the oxides allows the solder to flow and wet the surfaces to be joined. As heat exposure increases, the likelihood that a visible residue will remain around the solder bumps increases. Analysis of the residue on test boards has shown it to be comprised of tin compounds, perhaps a tin oxide matrix with tin salts of organic acids. The amount of oxide formed and the amount of metal salts formed upon fluxing increases with inc