Void-free, flux-free process for placement and attach of solder balls
02/01/2000
Technique increases production yields of wafers, flip chips, CSPs and µBGA packages.
RICHARD RAMOS
As production of wafer bumping, chip scale packages (CSPs), micro ball grid array (µBGA) and flip chip packages increases, so does the need for cost-effective and high yield production. The present method can produce unacceptable yields because it requires the use of flux; when the flux liquifies during the reflow soldering operations, the bonds between the flux, balls and pads are destroyed and the balls are free to move. Defects, such as vacancies, bridging of adjacent balls, loss of positional accuracy and voids, are thus created. Flux residues are also not always completely removed during post-solder cleaning processes because of voids, and high resistance shorts and/or corrosion may result.
To keep pace with the growing demand for flip chip technology and ball grid array (BGA) devices, high manufacturing yields are vital, and these yields must be improved to reduce manufacturing costs.
Federal Regulations
Many manufacturers have switched from solvent cleaning to aqueous saponification of rosin flux; many others are changing to water soluble organic flux. The primary considerations have been the continually rising costs of the solvents themselves, and to comply with Federal Environmental Protection Agency (EPA) and Occupational Safety and Health Agency (OSHA) regulations. The EPA has stringent categorical regulations on aqueous process effluents, as well.
There have been some significant changes in the past three years in how the EPA looks at the effluent from flux cleaning machines used by semiconductor manufacturers and contract assembly houses. City municipalities, such as San Jose and Santa Clara in California, have refused to allow the effluent from these machines to be discharged into sewer systems. This has necessitated use of recycling flux cleaners, which require very expensive deionized water systems and heavy metal capture resins. Because these resins when exhausted contain hazardous amounts of toxic materials (lead), the EPA requires hazardous material handling and disposal permits, because nearly all of the companies involved are three-to-four times over the maximum allowable limits for daily heavy metal disposal.
Effluent Standards
For example, the pretreatment standards that must be met in disposing of effluents from wave soldering and aqueous cleaning of electronic assemblies are:
• Federal regulations — These are categorical standards for uniform waste water treatment throughout the United States (Table 1).
• Typical municipal regulations — Protection criteria for municipal waste water facilities, which are the direct interpretation of federal regulations at the local level (Table 2).
It is clear from the data of Tables 1-5 that lead, BOD5, COD and pH standards were violated in three cases studied (these cases are presented in Tables 3-5). In the last case, TSS, copper and zinc standards were also violated.
Flux-free Process
While there were not any viable answers to this flux problem, there is a flux-free BGA, CSP, flip chip and wafer ball attach process for Sn/Pb or Pb/Sn solders available.
This process requires that oxides be reduced. The dissociation pressure, PoM2 = PoA2 increases for all metals. Thus, for an atmosphere containing a given partial pressure of oxygen, there exists a critical temperature Tc, at which the boundary condition will apply, whereby:
PoM2 > PoA2
and the oxide will commence to dissociate spontaneously, according to thermodynamic theory.
BGA Packages
Fluxless, high-volume ball attachment is performed in a large-format, controlled-atmosphere furnace. The tooling for the machine is composed of an upper and lower plate machined from graphite material.
The mating top plate or alignment/ loading plate is made from two inseparable plates that serve to load and align the balls. The lower plate contains a series of hole patterns that correspond exactly to the specified pad pattern of the BGA packages. The upper plate contains the same hole pattern as the lower plate and serves as a spring loaded "shutter" during the loading process. Dowel pins are inserted into the carrier and are allowed to protrude above the plane of the top surface. The carrier cavities are loaded with BGA packages and the alignment/loading plate is made to mate with the carrier by way of the dowel pins.
During the loading process, balls are placed onto the alignment/loading plate and fall into the open holes of the shutter. The shutter is approximately half the thickness of the balls, which ensures that a single ball will fill each hole. Once the shutter is full, a drain hole is provided so excess balls can be poured off; at this point, the balls are released by pushing the spring loaded shutter. The alignment plate is thicker than the shutter, which allows the balls enough space to completely clear the shutter on the return stroke; as a result, the balls can be precisely located on the BGA pads.
The fully loaded carrier assemblies are then placed in handling frames. Loaded handling frames can be maneuvered manually or by robots. The sectioned configuration of a loaded handling frame is shown in Figure 1. The chamber contains three graphite platens, which are the heat-generating medium. After placing two loaded handling frames on each platen (Figure 2), the solder profile is programmed into the memory of a microprocessor. Table 6 shows a typical profile for attaching solder balls to BGA packages.
The lid of the chamber is then closed and the air withdrawn. The chamber is backfilled with an appropriate inert gas, and an electrical current is passed through the platens. Because the graphite material is a poor electrical conductor, the platens are rapidly heated to the soldering temperature. Heat is conducted into the carrier assemblies from the platens. A vacuum bake operation is usually included in the solder profile just before ramping to the soldering temperature (Table 6). Therefore, any contaminating volatiles, such as entrapped water moisture, can be eliminated.
Once the solder balls reach liquidous, the solder material reacts with the gold surface of the BGA pads. The tin constituent of the solder alloy consumes the gold, and the solder wets the underlying metallization. Upon cooling, the surface tension within the molten solder mass causes the solder to form a ball-shaped structure during solidification. The visual appearance of the solidified mass is not unlike the original spherical shape of the solder balls, except that the solder at the base of the structure forms a fillet around the entire perimeter of the pad. The fillet indicates that good wetting has occurred between the solder and pad.
Because the tooling locates and restrains solder balls on the pads to within 0.001", defects, such as vacancies, bridging and loss of positional accuracy, are avoided.
The soldering technique, called pressure variation, uses as an advantage the reduction of the oxides in a vacuum and the pressure differential to eliminate voids. This method can be used to make large-area joints using solder preforms that have void levels consistently below five percent. More commonly referred to as Boyle`s Law, where:
P1 = initial pressure
P2 = final pressure
V1 = initial void volume
V2 = final void volume
Solving the equation for final void volume (V2 = V1 (P1/P2): Therefore, the greater P2 is in relation to P1, the more effective the process.
Flip Chip and Wafer Applications
An effective fluxless, high-volume method for positioning and attaching solder balls to wafers makes use of controlled-atmosphere furnace equipment. Because many thousands of balls are attached to each individual wafer, precise positioning is of primary importance. For unequaled accuracy, balls under .001" are solder-plated then reflowed in the vacuum atmosphere to produce void-free, flux-free ball attach on .001" or smaller balls.
The lid of the chamber is closed and the balls are automatically soldered to the pads in much the same way as previously described for BGA packages. Hot carrier assemblies can be removed by hand from the chamber using quick-release handles and studs or chamber loading and unloading operations can be performed by robots.
Conclusion
The fluxless, high volume method for attaching solder balls to BGA packages and wafers can produce good-quality solder joints (Figures 3 and 4). The precision tooling locates and restrains the balls in the required position during processing. Vacancy, bridging and loss of positional accuracy defects can be nearly eliminated, resulting in maximized production yields. AP
Acknowledgments
Special thanks to Stan Bernstein (Alpha Metals, Jersey City, N.J.) and James Green (Environmental Management and Planning Engineering, Nashville, Tenn.) for their contributions to this paper.
RICHARD RAMOS, executive vice president and division general manager, can be contacted at Scientific Sealing Technology, 9801 Everest Street, Downey, CA 90242; 562-803-3361; Fax: 562-803-4043; E-mail: [email protected].
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Figure 1. Loaded handling frame cross-sectioned in the plane referenced in Figure 2. The captured solder balls are aligned with the BGA pads. The walls of the alignment plate holes prevent the balls from moving during processing.
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Figure 2. Two fully loaded holding frames positioned end-to-end on a platen.
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