DANIEL K. WARD
Flux is an interesting substance that has a long history. Its use can be traced back to the Bronze Age when brazing was developed to make weapons and jewelry. Commonly used rosin flux, in its natural form, is the “life blood” or sap of an evergreen tree. Today`s sophisticated synthetic fluxes are formulated from base chemicals. Flux enhances the bonding of similar or dissimilar metal surfaces during the soldering process; this enhancement is achieved by the flux chemically breaking down oxides on the surfaces to be soldered, and preventing other oxides from forming during the melting and formation of the metallurgical structure (solder joint) that joins the surfaces together. Proper fluxing is absolutely essential to form strong and reliable solder joints required by electronic assemblies.
During the past 10 years, most electronics manufacturers have changed from fluxes that require post-soldering cleaning to no-clean flux materials and processes. No-clean fluxes leave flux residues that are not necessary to clean from the surface of the soldered assembly before final packaging. Cleaning elimination is possible because of the low chemical activity of the residue left on the assembly and its inability to generate corrosion in most electronic product operating environments. Eliminating the cleaning process, an expensive and a potentially environmental polluting process, provided the major industry drive to move to no-clean fluxes.
The automotive electronics industry has also been able to institute no-clean soldering processes, but it must protect some electronic products in sealed or potted cases. Such packaging is necessary to prevent the intrusion of harsh environment chemicals and their combination with flux residues that cause corrosion during operation.
Benefits and Challenges
Beyond this brief tutorial on fluxing and soldering, there are specific issues to address regarding no-clean flux and the soldering of flip chips. The primary purpose of flux in soldering flip chips is the same as with any other component. Fluxing with the proper materials ensures the formation of a strong and reliable metallurgical bond between the IC, solder bump and substrate. A secondary purpose is to provide a “tacky” surface to hold the flip chip in place on its soldering pads at assembly until solder reflow occurs. After soldering, flux residue left on the soldered assembly has no inherent positive value. In fact, it can have potential negative effects on the flip chip structure.
It has been documented that flux residue on the surface of the IC and substrate, which are in intimate contact with the underfill, can cause a lack of adhesion and subsequent delamination of the underfill from the IC die and substrate, causing premature solder bump fatigue failure. A number of technical papers have been published on this topic. There are also a substantial number of papers that support that no-clean fluxes that do not produce delamination failures are available. I submit that the authors of both types of papers are correct. The real issue is the application environment of the product being designed. If it`s a consumer product, such as a cell phone or computer, flip chips can be assembled with no-clean fluxes and result in reliable product. If the assembly is an engine controller for automotive under-hood applications, the flux residues must be removed.
Reducing delamination: New fluxes are being formulated to reduce the delamination effects of flux residues. One approach is an epoxy flux that fluxes the soldering area during soldering, and then combines with the underfill material to form one inert material system during underfill cure. Another approach is a flux that sublimes during the soldering process and leaves virtually no residue to interact with the underfill. Still another formulation is to add “adhesion promoters” to no-clean formulations, thereby enhancing (or at least not degrading) underfill adhesion to the flux residues.
It is important to note that flux residues are only one possible cause for underfill delamination. Incompatible substrate soldermask and IC passivation with underfill materials can also cause delamination. Based on the materials selected for a particular design, it is possible to have all three factors cause delamination at the same time, so don`t blame all your problems on the flux.
Volume residue: It`s also important to consider residue issues from a purely volumetric standpoint. Flux residues under flip chips form mechanical barriers that can impede the capillary filling action of underfill, leaving voided areas under die. These areas can either be void of any material or filled with flux only. Depending on the type and location of these voids, they can facilitate early fatigue failure of solder bumps. In the future, as bump pitches become smaller and the gap between the die and substrate becomes tighter, flux residue will present more significant mechanical interference in the application of underfill.
In summary, the cleaning of flux residues from under flip chip assemblies is to prevent delamination of underfill from IC and substrate during product operational life. Residues have been shown to reduce adhesion and create voiding, which can cause premature failure of solder bumps in automotive applications. Flux is only one of the materials that needs to be selected properly to make a successful flip chip assembly; the interaction of all of the materials in an assembly must be understood and controlled to maximize assembly operational life.
DANIEL K. WARD is manager of advanced electronic packaging for Delphi Delco Electronics Systems, One Corporate Center, P.O. Box 9005, Mail Station: D-16, Kokomo, IN 46904-9005; 765-451-3093; Fax: 765-451-3115; E-mail: [email protected].