Standard Flip Chip – Bump on I/O

Our Standard Flip Chip (SFC) process, formerly known as the Flex-on-Cap (or FoC) process, was created in the mid-1960's by Delco for use in the automotive industry. Today, the process has 40 years and over a million bumped wafers behind it. This is the process to use when you need to place small bumps (less than 135µm) directly on the die I/O. Pitch capabilities in this process are 90µm or greater for a full array I/O design and 90µm or greater for a peripheral I/O design. Typically, the number of bumps per die ranges from 4 to 6000. The SFC process uses premixed solder paste for the solder bumps. This provides for outstanding control of the alloy composition across the entire wafer. Since the process is not limited to the bi-metal restraints of an electroplating process, tri-metal alloys (such as Sn/Ag/Cu) require nothing more than the selection of the proper tube of pre-mixed solder paste. As with all die processed with small bumps, these die will require the use of underfill during packaging.

To take advantage of this process flow, the device must meet some minimum I/O pad requirements (described below). If the device does not meet these minimum I/O pad requirements, take a look at the SFC Repassivation flow. If you are looking for bump heights greater than 135µm, take a look at the UltraCSP flow – the industry standard Wafer Level Package (WLP) process flow.

The SFC-Bump on I/O process requires the fewest process steps of any flow that FlipChip offers.

I/O requirements for the SFC-Bump on I/O process

Since the SFC-Bump on I/O process forms a bump directly on the device I/O, certain criteria must be followed to ensure a proper bump structure. The basic rule is what we call "The Golden Rule of Flip Chip", which is:

The UBM must overlap the I/O passivation opening by at least 7µm and the I/O final metal pad must extend at least 5µm past the end of the UBM.

Figure 1 shows the "Golden Rule" requirements for the UBM in relation to the passivation opening and the I/O metal bond pad. This rule requirement accomplishes several reliability requirements. The overlap of the UBM to the passivation opening provides a seal to the underlying I/O aluminum bond pad. The overlap of the UBM inside the I/O metal bond pad eliminates stresses that can cause silicon cratering.

Figure 1. The Golden Rule of Flip Chip

The allowable size of the UBM is directly related to I/O Pitch. The bump height is strongly influenced by UBM size.

If your device does not meet the requirements of the Golden Rule, your device may be a candidate for the SFC-Repassivation process, which is described in the next section.