Printed Circuit Design & Fab - November 2007 - (Page 30)

Plating ratio @ 50% Barrel Position Effect of Agitation 1 0.8 0.6 0.4 0.2 0 8 9 10 11 Relative Speed (ft/min) 12 13 Plating Ratio @ 50% Barrel Position PLATING Effect Of Free Stream Concentration 0.6 0.5 0.4 0.3 0.2 0.1 0 100 120 140 160 180 Concentration (grams/cubic foot) 200 220 FIGURE 8. Effect of agitation on plating ratio. FIGURE 9. Effect of free-stream concentration on plating ratio. k is a constant (plating bath efficiency) c is the local concentration c0 is the maximum concentration (concentration of the bath at large) Since the plating bath is usually very large compared to the cathode, the bulk concentration of the bath will be assumed to be constant at c0. The concentration of the plating solution in the through hole, however, decreases as the plating fluid transverses the hole. The analysis requires an algebraic expression for this mechanism. The relationship between the position in the barrel (x/l) and the concentration becomes: 1 − exp(−kc / c 0 ) π D 2 (V0 ) 2 c0 c x () [ − 1 + k ln( )] (Eq. 11) =− • υ ( w) c0 256 A 1 − exp(−k) l s Plated Through-Hole Dynamics In FIGURE 4, a control volume of plating fluid is shown moving through the PTH at a speed of u and plating the surface • of the barrel at a local rate w . To an observer attached to the control volume, the rate of change of the concentration • in the control volume ( c ) is: c= • Equation 11 defines the concentration along the barrel of the PTH; unfortunately, however, the relationship is implicit. That is, one cannot solve directly for the concentration and, consequently, a numerical technique is needed. With the concentration defined, the plating efficient follows (see Equation 8). Then using Faraday’s Law (Equation 7), it is now possible to calculate the theoretical plating thickness anywhere in the barrel. It will be noticed in Equation 11 the controlling parameters are: (D/A), V0, c0, and (w)s • dc GI u=− dx V (Eq. 9) where V is volume of the control volume. Combining Equations 7, 8 and 9 and then integrating using the identity that c=c0 at x=0, the result is a relationship between the concentration and the position in the barrel. kc 1 − exp( − ) c0 c0 x =− {( c − c 0 ) + ln ( )} • 1 − exp( − k ) k ( w )s uV • (Eq. 10) where ( w )s is the plating rate at the surface of the cathode. The velocity (u) of the control volume is given by Equation 3. Also it is pointed out in the National Metals Finishing Resource Bluebook that the plating efficiency of a copper bath is approximately 10% when the concentration is 2.86 g/ft3. In that case, the value of k is: 1 I χ = - ln(1 − ) c I0 The plating bath index, which can be influenced by plating bath additives such as “levelers,” is defined as: k= c0 I ln(1 − ) c I0 In this case x = 0.0367 k = 0.0367c0 30 It has long been empirically realized that plating becomes more difficult with increasing hole aspect ratio, but Equation 11 shows that the hole diameter plays an equally important role separate and apart from the aspect ratio. It is also common knowledge that through-hole plating is improved with agitation. Equation 11, in fact, finds the agitation speed plays a super linear role. The concentration of the bath is also important, as is the plating rate at the surface. The surfaceplating rate is principally a function of the applied current I0, which demonstrates that low amperage plating will improve the uniformity of PTH side wall, which has recently been the observation of several leading-edge PCB shops. The plating profiles are demonstrated as a function of these variables. This will be accomplished by using variations from the Based Line Parameters shown in the TABLE 1. The first variation shown in FIGURE 5 looks at the effect of aspect ratio and hole diameter. As the aspect ratio is increased, the plating thickness along the hole-wall is reduced in a nonlinear manner. It is also noticed that as the hole diameter decreases, the plating thickness quickly subsides causing a substantial nonlinear variation in the thickness of copper along the barrel. For instance, at an aspect ratio of 6, the impact of the hole diameter is minor, but at an aspect ratio of 14, the impact is severe. This information is now summarized by viewing the minimal through-hole plating thickness, which occurs halfway through the barrel after being normalized to the surface plating rate. A value of 1.0 is the desirable result. NOVEMBER 2007 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - November 2007

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