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 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 increased heat exposure.
The prevention of white residues takes on two highly important process steps: proper heat management from the reflow process, and optimization of the cleaning fluid to adequately remove the residues during the initial cleaning cycle. The organic layer dissolves quickly in the ACC cleaning solvent system, dislodging the inert residues. An aggressive solvent action is necessary to break up the oxide layer at the same time the organic layer is removed. Enhanced fluorinated cleaning fluid with a higher concentration of active solvent is necessary to completely remove the inert layer along the organic layer. If residues remain after cleaning, future cleaning cycles will not remove them.
Cleaning Fluid Enhancement
The current ACC solvent blend is composed of three active solvents, and the balance is fluorinated materials and inhibitors. Only one of the active solvents is the primary solvating agent and higher concentration is required to effectively dissolve difficult residues. Previous fluorinated solvents have been introduced that azeotrope with the solvating agent at higher levels. This enhanced solvent composition is significantly higher in active solvents required to remove white residue around solder.
Board assemblies were tested for residual rosin concentration by UV vis. at various active solvent concentrations per IPC test methods. Increasing the active solvent substantially reduces the residual resin after the cleaning step.
Fluorinated materials are relatively inert and mild, with selective solvency for soils, oils and greases. The materials exhibit limited solvency for soils, oils and greases. The materials also exhibit limited solvency for many higher molecular weight materials such as rosin and synthetic resin-based flux residues. To address the lack of solvency, the chemistry of the cleaning fluid must be similarly adjusted by engineering a composition that forms azeotrope-like properties at higher active solvent levels. These materials can be effectively blended with a variety of other solvents to form specialized cleaning agents for cleaning the challenges of today's electronic assembly environment.
The engineered material composition must have properties that would enhance the performance for removing the flux residues, as well as acceptable environmental and toxicity profiles. The engineered composition must contain the proper ratio and balance to form azeotropes or behave like azeotropes. For clarification, an azeotrope is a mixture of two or more components that vaporizes without a change in composition at the boiling point, and exhibits a boiling point less than any of the individual components. After evaporation, only a small difference exists between the composition of the vapor and the composition of the initial liquid phase. This difference is such that the compositions of the vapor and liquid phases are considered substantially the same.
The most effective active solvent is 1, 2-trans-dichloroethylene (DCE), which has favorable toxicological, environmental and performance properties. Proprietary fluorinated material forms an azeotrope with DCE at higher levels. This property allows the formulation of solvent blends that provides enhanced performance on the removal of many RMA, RA and low-residue resin flux residues. The resultant fluids can be formulated to have no ozone depletion, while maintaining superior cleaning ability, drying ability, low viscosity and low surface tension. In addition, desired blends will exhibit no flash point in keeping with the CFC and chlorinated-based solvents.
When building engineered fluid compositions, flash point must be a consideration. Oxygenated solvents are needed to enhance cleaning and removal of ionic contaminants. When blending oxygenated compounds within the composition, flash point is a concern. To address this concern, a more highly fluorinated compound may be added to the ternary azeotrope-like blend thus retarding the flashpoint. The addition of this highly fluorinated compound forms a unique blend of azeotropes. Being substantially constant boiling, the compositions do not tend to fractionate to any great extent upon evaporation. Since the mixtures are not easily fractionated, they are useful commercially in standard cleaning apparatuses for cold cleaning or vapor degreasing. The composition of the vapor and liquid phases are considered substantially the same and are either azeotropic or azeotrope-like in their behavior, which is a blend that is suitable for commercial use.
To better understand performance, the soil must be evaluated for suitability with the cleaning technology. Solder flux is an important engineered material that enhances solderability of electronic circuitry. A solder flux needs to perform a number of important functions at the same time. It must promote thermal transfer to the area of the solder joint, enhance wetting of the solder on the base metal, and prevent oxidation of the metal surfaces at soldering temperatures. Among those, the primary task is to remove the tarnish layer form the metal joint that is about to be soldered. The most commonly used fluxes for solder pastes include organic acids, organic bases, organic halogen compounds and organic halide salts.
The ingredients in flux are complicated. In general, fluxes comprise resins, activators, solvents and rheological additives. For certain special systems, additives such as tackifiers, surfactants, or corrosion inhibitors may also be used.
Resin refers to organic materials with medium to high molecular weight. It may include natural products, such as rosin, or synthetic materials, such as polymers. Often it is used to provide fluxing activity, tackiness, and an oxygen barrier. Sometimes it may also serve as a rheological aid. The most commonly used resins are water-white rosin or chemically modified resins. The latter type is sometimes referred to as synthetic rosin or synthetic resin by the soldering industry. These chemically modified materials can undergo polymerization, hydrogenation, or functional group modification. These changes may increase the difficulty of cleaning the residue after soldering.
Although resin may provide certain fluxing activity, the soldering performance of resin alone is rarely good enough for the electronics industry. Often some activator chemicals have to be added to the flux in order to boost fluxing activity. The most commonly used activators include linear dicarboxylic acids, special carboxylic acids, and organic halide salts. Activators with a greater solubility in water, such as glutaric acid and citric acid, generally are more suitable for aqueous cleaning processes. Low residue no-clean activators such as adipic acid, may or may not be cleaned in a solvent system.
Figure 4. (left) Increased active solvent. (right) Lower levels of active solvent.
Visual inspection of test boards revealed that increased levels of the active solvent enhance cleaning. With higher levels of active solvent, solubility of the residue improves. This provides an added benefit of reduced cycle time. Figure 4a is a visual representation of flip chip sites that represent improved cleaning. Figure 4b is a representation of residue when using lower levels of active solvent.
Figure 5. Solvent concentration effect.
Active Solvent Level
One further experiment demonstrates the solubility characteristics of the white residue in the fluorinated family of solvents. Samples of white residue were collected from the boil tank of a vapor degreaser. A small quantity of residue was dissolved in a mixture of 85 percent active solvent and fluorinated molecule. The entire residue was soluble in this mixture. Following this, the mixture was diluted with fluorinated molecule to reduce the active solvent concentration from 85 to 56 percent. The residue remained in solution. Similarly, an equally sized sample of residue was mixed with ACC product (43 percent active solvent). In this case, the residue was not totally soluble in the mixture. Additional active solvent was added to increase the concentration from 43 to 72 percent. During this time, no further dissolution of residue was observed (Figure 5). This effect highlights the importance of active solvent concentration for effective cleaning.
Organic flux residues deposited onto the surface of flip chip assemblies during reflow soldering operations have been shown, in some cases, to affect the properties of underfill. Formation of metal salts is due to the active ingredients in the flux chemistry. White residues that form as a result of solder/flux interactions are difficult to completely remove. Increased active solvent concentration in cleaning fluids can greatly improve cleaning performance.
The active solvent concentration is an essential driver for cleaning. A proprietary fluorinated material has been identified that azeotropes with the active solvent at higher level. Visual inspection of test boards revealed that increased level of active solvent, solubility of the residue improves.
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MIKE BIXENMAN, chief technology officer, and KYLE DOYEL, president, may be contacted at Kyzen Corp., 430 Harding Industrial Dr., Nashville, TN 37211; 615/831-0800; e-mail: email@example.com and firstname.lastname@example.org.