Active Solvent Concentration is Essential Driver for Cleaning
BY MIKE BIXENMAN AND KYLE DOYEL
On flip chip assemblies using capillary underfill, flux residue remaining after reflow must be removed to allow the uniform flow of underfill and to ensure a strong adhesive bond line between the flip chip and board. A common method to remove residue is by dissolution in a vapor degreaser. Alternate, non-ozone-depleting solvents have become available since the Montreal Protocol's elimination of nearly all previous chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). A solvent must be selected based on several factors, including: cleaning performance, safety, regulatory restrictions and cost. These factors can have different degrees of importance in a given situation. The best-performing solvents may pose toxicological, environmental or flammability concerns. Typically, solvent mixtures operate at a stable azeotropic composition in a vapor degreaser. This article discusses how a newly discovered azeotrope-like mixture with improved performance can be a viable process alternative.
New vapor degreasing solvents are more costly than their ozone-depleting predecessors. This has driven the development of cleaning systems with low evaporative and drag-out losses. Design improvements include subzero chillers and extended freeboard height. These modifications have greatly reduced solvent emissions.
Solvent cleaning is a combination of two mechanisms. The first is dissolution, where the soils are dissolved in the solvent and carried away. Condensate of the solvent in the degreaser continually replenishes the dirty solvent with fresh, clean solvent. The second mechanism is mechanical displacement of insoluble components. This can be achieved by spray impingement, turbulence or ultrasonic agitation. For flip chip assemblies, the use of ultrasonic energy has been avoided to reduce the risk of damaging small flip chip solder joints.
Figure 1. Example of a batch cleaner.
Two basic equipment configurations are used to clean 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