Shared via decoupling for area arrays components
A solution for mounting decoupling capacitors on a printed wiring board (PWB) used for mounting a high performance ball grid array (BGA) device is described. The via array that connects the BGA device is modified, the modification being that at least a portion of one row of said vias array is missing at least two adjacent vias. The missing vias are replaced by respective shared vias in an adjacent row, and the shared vias are connected to either a power supply or a power return. The shared vias are also provided with via pads on the other side of said PWB, and a decoupling capacitor can be electrically connected across the pair of via pads to decouple the power supply and the power return at the two adjacent vias.
1. A printed wiring board (PWB) for mounting a high performance integrated circuit, comprising:
on a top side of said PWB, a modified via array with BGA columns and BGA row of ball connection pads;
a modified vias array of plated through hole (PTH) vias, with each via column Col(n) arranged between two respective BGA columns c(n) and c(n+1) and each via row R(k) arranged between two respective BGA rows r(k) and r(k−1), wherein 2m vias of said via column Col(i) placed in successive via rows R(j) to R(j+2m−1) of said modified vias array are depopulated to obtain a free space on the back side of said PWB, wherein 2m corresponding vias in a via column Col(i+1) adjacent to said via column Col(i) and placed in said successive via rows R(j) to R(j+2m−1) of said vias array are shared vias, wherein n≧1, k≧2, 2≦i<n and j<k, and wherein a first shared via in said via column Col(i+1) and said via row R(j) provides a power contact to a first associated ball contact pad in said via column c(i+1) and said via row r(j) and to a second associated ball contact pad in said via column c(i+2) and said via row r(j).
2. The PWB of claim 1 , wherein said free space has a width D 1 equal to twice the pitch D of said vias array less a via size, for accommodating m passive elements of a substantially similar width D 1 .
3. The PWB of claim 2 , wherein m is at least one.
4. The PWB of claim 2 , wherein said passive elements are capacitors.
5. The PWB of claim 4 , wherein the capacitors have a size of substantially 0.06″×0.03″.
6. The PWB of claim 4 , wherein the capacitors have a size of substantially 0.04″×0.02″.
7. The PWB of claim 4 , wherein the capacitors have a size of substantially 0.02″×0.01″.
8. The PWB of claim 2 , wherein said passive elements are resistors.
9. The PWB of claim 8 , wherein the resistors have a size of substantially 0.06″×0.03″.
10. The PWB of claim 8 , wherein the resistors have a size of substantially 0.04″×0.02″.
11. The PWB of claim 8 , wherein the resistors have a size of substantially 0.02″×0.01″.
12. The PWB of claim 1 , wherein a second shared via in said via column Col(i+1) and said via row R(j+1) provides a ground contact to a third associated ball contact pad in said via column c(i+1) and said via row r(j+1) and to a fourth associated ball contact pad in said via column c(i+2) and said via row r(j).