Patent Grant
7244331
The present invention is directed toward a method of producing a low-temperature, cofired ceramic (LTCC) substrate with cavities having improved bondability, and, more specifically, toward a method of producing an LTCC substrate having cavities, using a stretchable sheet of material to transfer pressure to the cavities, which cavities have a bondability similar to the bondability of the surrounding substrate.
Low temperature cofired ceramic (LTCC) substrates used in connection with electronic circuitry, and more particularly integrated circuits, are generally well known and typically comprise a multilayered structure of ceramic material which includes stripline circuit patterns and openings therethrough called vias for providing circuit interconnection between layers. The layers are laminated together by externally applied pressure from, for example, a hydrostatic press. Such a press may apply pressure on the order of 250 to 6000 psi or higher to the laminate components.
Many such substrates have cavities in their surfaces. To apply pressure evenly to the cavities, silicone molds having a shape corresponding to the cavities may be placed in the cavities. As the cavities have become more numerous and complex in shape, having, for example, internal ledges and other non-uniform features, the silicone molds have become more complex as well and therefore more expensive.
One method for forming LTCC substrates without using silicone molds is disclosed in U.S. Pat. No. 5,683,535, entitled “Method and Apparatus of Producing Cavities in LTCC Substrates,” the entire contents of which are hereby incorporated by reference. A method similar to the one described in that patent is outlined below.
In order to protect the corners of the cavities 24 and to reduce rounding of the edges thereof during the lamination process, a single sheet 36 of 0.008 in. thick talc coated latex rubber is placed over the template. Next, pairs of 0.125 in. thick silicone foam sheets 28 and 30 are trimmed to the required area of the parts and are placed above and below the assembly including the tooling plate 14, template 20, and silicone mold 26. The assembly is then placed in a flexible vacuum bag, shown schematically by reference numeral 32. The silicone rubber sheets 28 and 30 act as a protective barrier to keep the vacuum bag 32 from being punctured by a rigid edge of the lamination stack-up assembly located therebetween.
The vacuum bag 32 and its contents are then placed in a hydrostatic pressure chamber shown by the phantom lines 34. The hydrostatic pressure chamber 34 generates a pressure of between about 250 psi and 6000 psi which isostatically laminates the LTCC sheets 12 together, forming a composite substrate structure.
Metallic wires, ribbons and similar elements are sometimes attached to a substrate after the lamination process. The bonds between these elements and the substrate must be sufficiently strong to prevent separation during normal use. It has been found, however, that the bondablity of the cavity interiors is generally less than the bondability of the substrate surfaces surrounding the cavities. This difference is particularly noticeable in substrates made according to the process disclosed in U.S. Pat. No. 5,683,535. It would therefore be desirable to provide a method of producing an LTCC substrate having cavities that avoids the need for separate molds in each cavity while at the same time producing cavities having acceptable levels of bondability.
These problems and others are addressed by the present invention which comprises, in a first aspect, a method of producing an LTCC substrate having improved cavity bondability that involves providing a stack of green ceramic tape sheets having a cavity and a template having an opening corresponding to the opening in the stack. The template is placed over the stack with the opening aligned with the cavity. A sheet of stretchable material having graphite applied thereto is placed over the template with the graphite toward the template, and the tape sheets, template and sheet of stretchable material are isostatically laminated to produce an LTCC substrate having a cavity. Then the template and sheet of stretchable material are removed from the stack.
Another aspect of the invention comprises a method of producing an LTCC substrate having improved cavity bondability that involves providing a stack of green ceramic tape sheets having a cavity and a template having an opening corresponding to the opening in the stack. The template is placed over the stack with the opening aligned with the cavity. A sheet of stretchable material having zinc stearate applied thereto is placed over the template with the zinc stearate toward the template, and the tape sheets, template and sheet of stretchable material are isostatically laminated to produce an LTCC substrate having a cavity. Then the template and sheet of stretchable material are removed from the stack.
A further aspect of the invention comprises a method of producing an LTCC substrate having improved cavity bondability that involves providing a stack of green ceramic tape sheets having a cavity and applying zinc stearate to the stack. Next, a template having an opening corresponding to the opening in the stack is placed over the stack with the opening aligned with the cavity. Then a sheet of stretchable material is placed on the template, and the tape sheets, template and stretchable sheet are isostatically laminated to produce an LTCC substrate having a cavity. The template and sheet of stretchable material are then removed from the stack.
These aspects of the invention and others will be better understood after a reading of the following detailed description in connection with the following drawings, wherein:
It would appear initially that all parts of the substrate are exposed to the same conditions during lamination. Therefore, various aspects of the substrate manufacturing process, from the laser cutting of the cavities in the LTCC sheets to the stacking and lamination thereof, had to be considered to determine why the cavities exhibited lower bondability.
As a first step in determining why the cavities exhibited a lower bondability that the rest of the substrate, the substrates made by the above conventional process were analyzed with scanning electron microscopy and Auger spectroscopy. These analyses revealed that magnesium and silicon contaminants were present in the cavities, but not on the substrate regions between the cavities. Because the talc coated latex contacted the cavity interiors during lamination, but did not contact the regions between the cavities, it appeared that the contaminants were residues from the talc release agent used in the above process. It was therefore theorized that these contaminants could be affecting the bondability of the cavity interiors. Different release agents were therefore tested to determine whether 1) their use would eliminate the magnesium and silicon residues and 2) whether the removal of such residues would improve bondability.
It was found that graphite, in the form of a powder or an aerosol spray, applied to the flexible sheet 36 substantially prevented sticking when the latex sheet 36 is removed from the substrate. At the same time, the graphite did not affect the subsequent bondability of the substrate. Likewise, zinc stearate, applied as a powder or a spray, substantially prevented the latex sheet from sticking to the substrate. The zinc stearate also did not adversely affect the bondability of the substrate. Beneficially, the zinc stearate may be applied directly to the substrate instead of the sheet 36 if desired. While the graphite would also prevent sticking if applied directly to the substrate, it could potentially also damage the substrate and thus this method of graphite application is not recommended.
Table 1 below illustrates the results of a 1 mil wire bondability study for substrates produced using 1) uncoated latex 2) latex coated with zinc stearate 3) latex coated with graphite and 4) silicone coated with graphite. The graphite coated silicone did not perform satisfactorily, and it was later determined that the silicone had a talc coating that presumably produced these unsatisfactory results. While talc-less silicone was not tested, it appears likely that it would provide acceptable bondability.
The latex used was obtained from AirTech Advanced Materials Group, product designation SL200, having an elongation of 500+% and a maximum use temperature of 121 degrees C. While uncoated latex did not affect cavity bondability, it stuck to the substrate and elements, including metallic elements formed thereon, thus damaging the substrate. The requirement for satisfactory bondability is that the mean force required to separate the wire from the substrate be at least three standard deviations greater than a minimum force, in this case, 3 grams, and that no “no sticks” occur. As can be seen from the above data, graphite coated latex and zinc stearate coated latex both produced cavities having acceptable bondability levels.
Table 2 illustrates the results of a second 1 mil wire bondability study.
As was the case with Example 1 above, these data demonstrate that the bondability of substrate cavities produced using a stretchable sheet of material to transfer pressure to cavity interiors can be improved when graphite or zinc stearate is used as a release agent instead of the conventionally used talc.
Table 3 illustrates the results of a bondability study using a 0.5×2.0 mil ribbon.
Acceptable bondability in the above ribbon test required that the mean force required to separate the ribbon from the substrate be at least three standard deviations above 3.5 grams. The above data show acceptable bondability in the above case as well.
The present invention has been described herein in terms of several embodiments, it being understood that obvious modifications and additions to these embodiments will become apparent to those skilled in the relevant arts upon a reading of this disclosure. It is intended that all such modifications and additions comprises a part of this invention to the extent that they fall within the scope of the several claims appended hereto.
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