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Effect of Soldering Flux It has been shown that polyglycols12 (which are found in water-soluble fluxes) diffuse into the epoxy during soldering.13 This absorption has been shown to reduce electrical performance by increasing moisture uptake by the substrate.14 Jachim et al.15 were the first to link the use of polyglycols in soldering fluxes and fusing fluids to increased susceptibility to CAF formation. Ready et al.16 showed that the use of certain water-soluble fluxes or fusing (hot air solder level [HASL]) fluids could increase CAF formation. In examining a catastrophic field failure (see Fig. 56.9), Ready et al.16 demonstrated the presence of a copper-bromide containing salt between an innerlayer power plane and ground pin, separated by 0.005 in. Using a test coupon manufactured in the same lot as the failed product, they extracted the flux residues from an innerlayer of the multilayer board and used ion chromatography to match the residues from the board s innerlayer with the constituents of the flux.
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FIGURE 56.9 Catastrophic field failure of military hardware. The conductive filament grew from the +20 V ground plane to the 20 V ground pin. Flux residues enhanced this failure rate.16
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The morphology of CAF is different when certain fluxes are used.6 In Fig. 56.10(a), when no flux is used, CAF forms only as a crystalline filament at the epoxy-glass interface; in Fig. 56.10(b), the flux contained polyethylene propylene glycol (1800) and there appears to be copper-containing compounds in stratified layers within the matrix in addition to the filament at the epoxy-glass interface; and in Fig. 56.10(c), the flux contained a linear aliphatic polyether and one sees a copper-containing compound appearing in a striated morphology as well as in the filament at the interface.
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Humidity in the PWB Storage and Use Environment J.A.Augis et al.17 determined that there is a humidity threshold below which CAF formation will not occur. They found that this relative humidity threshold depends on operating voltage and temperature. It is important to remember that this relative humidity need not be present in the operating environment. Moisture absorption can occur during any part of the assembly s lifetime.
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FIGURE 56.10 Morphology of CAF differs when different flux constituents are used: (a) CAF only at the glass-epoxy interface; (b), (c) an additional copper-containing compound within the polymer matrix.
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This is particularly critical during transportation or storage, when the assembly may experience harsh environmental conditions.
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TEST METHOD FOR CAF-RESISTANT MATERIALS
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In the late 1990s, laminate suppliers began to develop new materials that they marketed as CAF-resistant. To evaluate these materials, K. Sauter18 developed a CAF test vehicle, which consisted of a multilayer board with daisy-chained hole-to-hole spacing of 0.25 mm, 0.375 mm, 0.50 mm, and 0.625 mm.The holes were either inline with the glass fiber direction or staggered. Accelerated aging was done at 65 C and 85 percent RH for 500 hours. Sauter s results show variations based on laminate material, manufacturer, and diagonal versus inline holes, with the former being more CAF-resistant. He also defined a readily conductive region around the plated-through holes, which must be considered in establishing design rules. This test vehicle and procedure has been developed as an IPC Test Method (IPC-TM-2.6.25), Conductive Anodic Filament (CAF) Resistance. 19
MANUFACTURING TOLERANCE CONSIDERATIONS
Test board designs for performing statistical studies of CAF-resistant materials must take into account the limitations of the PWB manufacturing process.20 As hole-to-hole spacing becomes closer, the limits of the drilling process must be considered. Drill wander can be caused by splay when the drill enters the board at an angle. These two factors are exacerbated with smaller drill size. Typical manufacturing tolerances for hole-drilling equipment are specified at 0.075 mm. Based on this, test board design for statistical CAF studies should have the minimum edge-to-edge spacing of 0.375 mm, since the manufacturing tolerance should be no more than 20 percent of the hole-to-hole distance. CAF-resistant substrates are frequently more expensive than traditional FR-4, and therefore they are reserved for products that require high reliability, optimizing both cost and performance. Designs are multilayer rather than double-sided rigid boards. An important difference between a multilayer board and a double-sided rigid board is the glass-to-resin ratio. In drilling the holes, more damage will occur at the epoxy-glass interface for boards with higher glass content. Typical double-sided PWBs use eight sheets of 7628-glass cloth with a thickness of 0.175 mm (0.007 in.) to create a 1.5 mm (0.062 in.) thick board. The portion of resin in this board would be 30 to 40 percent. For a multilayer board, the glass cloth thickness may range from 0.035 mm to 0.10 mm (0.001 in. to 0.004 in.) per layer. In this case, the amount of resin ranges from 45 to 50 percent in the case of the 0.10 mm glass cloth thickness, it ranges from 55 to 65 percent in the case of the 0.035 mm thickness. In the drilling process, the board with a higher resin content creates less hole damage than one with a lower resin content. Microvias that are created by laser drilling can be located at a reduced pitch for CAF studies because one does not expect drill damage from a laser-ablated hole and it will not be subjected to the drill wander of a mechanically drilled hole. Thus, laser drill tolerances are below 0.025 mm (0.001 in.). The catastrophic failure presented earlier in this chap. (see Fig. 56.9) was caused by a power plane (anode), which created a CAF that shorted to the hole (cathode). In designing a test board, the power plane must always be the anode in order to reduce the current density for corrosion. A large cathode in contact with a small anode creates a high-current density at the anode and increases the corrosion rate. Present BGA board designs have microvias of 1 mm (0.040 in.) or even 0.8 mm (0.032 in.). These become problematic when the plane-to-hole separation is below 0.2 mm (0.008 in.).
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