Electronic packaging generally contains many levels of packages and interconnections. A first level package may connect one or more silicon chips on a ceramic substrate carrier. A second level package may interconnect one or more such ceramic substrate carriers on an organic board.
The ceramic substrate is connected to the organic board by pins which are typically rigid. The rigid pins are brazed on the ceramic substrate with a suitable braze material such as a gold-tin alloy. Ceramic substrates with such pin grid arrays (PGA) are subsequently plugged into a connector or wave soldered to an array of plated through holes on the organic board. This connection system has disadvantages such as the through holes limiting the number of wiring channels available in the board. Another disadvantage is the high cost associated with the braze material, the rigid metal pins, and the pin connectors or plated through holes.
As disclosed in U.S. Pat. No. 4,914,814, these disadvantages can be avoided by using solder column connection (SCC) technology which is also known as CCGA (ceramic column grid array) technology. SCC technology has a lower cost than PGA technology. SCC technology also provides an improved electrical interconnection that can better withstand the stresses associated with thermal expansion mismatch between a ceramic carrier and a supporting circuit board.
Referring to FIG. 1, a ceramic carrier 100 may have one or more semiconductor chips 160 attached thereon. To connect a ceramic carrier 100 to a supporting circuit board 110 using SCC technology, solder columns 120, typically 90% lead/10% tin, are used. The solder columns 120 are first formed and then attached to the ceramic carrier 100, then the end of the solder columns 120 opposite the ceramic carrier 100 is attached to the circuit board 110.
Referring to FIG. 2, the solder columns 120 of FIG. 1 are formed and then attached to the substrate 100 by using a graphite mold 200. The mold 200 has an array of pin holes 210. The pin holes 210 of the mold 200 are filled with solder segments 220. The ceramic carrier 100 is aligned (X, Y and radial adjustment) so its conductive I/O pads 130 are in alignment with the ends of solder segments 220 in the pin holes 210 of the graphite mold 200. Flux is applied on the substrate 100 surface and the substrate and solder segments 220 are assembled together. The assembly is processed through a furnace which joins the solder segments 220 to the conductive I/O pads 130 of the ceramic carrier 100, resulting in the formation of cast solder columns 120 of FIG. 1.
Referring to FIG. 1, to attach the ceramic substrate 100 with the cast solder columns 120 to an organic board 110, eutectic solder paste 150 is applied to metallized pads 140 of the organic board 110. The ends of the solder columns 120 opposite the substrate 100 are aligned with the metallized pads 140 and they are passed through a furnace which melts the eutectic solder paste 150 and bonds the metallized pads 140 and the solder columns 120.
A problem associated with the CCGA process is caused by poor solder quality. The composition of incoming solder used in the mold 200 to create solder columns 120 can vary from the pure-composition of 90% lead/10% tin. Solder varying in composition from pure-composition solder is referred to as off-composition solder. Off-composition solder having a lower lead content than pure-composition solder has a lower melting point than pure-composition solder. The melting point of pure-composition solder is approximately 300.degree. C. The melting point of off-composition solder columns typically ranges from 183.degree. C. to 220.degree. C. The exact melting point of off-composition cast columns depends on their lead/tin alloy composition.
The use of off-composition solder columns to attach a substrate 100 to a board 110 can result in manufacturing defects. When a substrate 100 with off-composition columns is attached to an organic board, during the reflowing of the eutectic solder paste 150 on the board 110, the off-composition columns are partially melted. This can result in several defects post module/card assembly. These defects include the columns being hourglass shaped and/or having a small solder fillet at the card side of the column. These defects result in possible loss or expensive rework at the card level assembly.