BGA package with traces for plating pads under the chip
A semiconductor flip-chip ball grid array package ( 600 ) with one-metal-layered substrate. The sites ( 611 ) of a two-dimensional array become usable for attaching solder balls of the signal (non-common net assignment) I/O type to the substrate under the chip area ( 601 ), when the sites can be routed for metal plating ( 620 ). The space to place a maximum number ( 614 ) of signal routing traces is opened up by interrupting the periodicity of the site array from the edge ( 602 ) of the substrate towards the center under the chip. The periodicity is preferably interrupted by depopulating entire aligned lines and rows of the two-dimensional array.
1. A method for fabricating an electronic device comprising the steps of:
opening n times m through-holes at the center of a substrate forming a n-row-by-m-column first array of through-holes at a first pitch, where n and m are integers no smaller than 3;
opening through-holes adjacent to the first array extending towards edges of the substrate forming a second array of through-holes, including the first array of through-holes, at the first pitch;
forming metal lands on a first substrate surface covering the through-holes;
forming a metal trace on the first substrate surface extending from each metal land; and
placing a plurality of the traces on the first substrate surface between at least two adjacent pairs of through-holes of the second array of through-holes where the two through-holes of each of the at least two adjacent through-hole pairs are spaced multiple first pitches apart from each other.
2. The method of claim 1 , further comprising opening through-holes at the edges of the substrate forming a third two-dimensional array of through-holes including at least three rows of through-holes parallel to each edge.
3. The method of claim 2 , further comprising forming an elongated gap free of through-holes therein and perpendicular to an edge of the substrate, wherein through-holes of the third array adjacent to the gap are spaced multiple second pitches apart from each other.
4. The method of claim 3 , further comprising placing the plurality of the traces across the elongated gap.
5. The method of claim 4 , further comprising terminating the plurality of the traces at an edge of the substrate.
6. The method of claim 1 , in which the metal lands and the metal traces are formed on a first surface of the substrate.
7. The method of claim 2 , further comprising forming metal lands covering the through-holes of the third array of through-holes.
8. The method of claim 2 , further comprising forming metal traces extending from the metal land covering the through-holes of the third array of through-holes.
9. The method of claim 6 , further comprising flip bonding a semiconductor chip on the first surface of the substrate.
10. The method of claim 7 , in which the flip bonding comprising forming metallurgical bonds between the semiconductor chip and the metal traces.
11. The method of claim 7 , in which the semiconductor chip covers the second array of through holes.
12. The method of claim 6 , in which the substrate has a second surface opposite the first surface and free of metal lands and metal traces.
13. The method of claim 1 , in which n is equal to m.