Tutorial #94
Terence Q. Collier
May 2009
Bumping single die or partial wafers is a quick and economical solution for development, prototyping, small production builds and product redesigns. Individual bare die, partial wafers, or complete wafers can be bumped. We can also remove die from chips, packages, or modules to re-ball them for new applications.
Bumping options include various solder alloys, nickel-gold or copper-gold pillars, or gold stud bumps. Any of these can be supplied at a fraction of the cost of bumping a complete wafer, shortening the cycle time by weeks or months compared to wafer bumping.
CAPABILITIES
What types of product can we bump? Choices range from single 0.5mm die to complete 300mm wafers. Inorganic and organic substrates, PCB’s and modules can also be bumped. Bumping for die-on- die assembly is also available.
We do offer a suggestion about full wafers. Often only a single wafer is available. Separating it into partial wafers allows us to process and validate multiple bumping solutions. A partial wafer lets you evaluate a few die and save the remainder. We also offer the option of testing die with different bump alloys on a single wafer.
With our single die capability, CVI can create dummy die from various materials to evaluate the effects of solder alloy, die pad size, wetting and post reflow gap heights on underfill reflow.
CVI also offers redistribution layers and bumping on partial wafers as small as 1cm square. Since we can bump single die, we can also repair bumped die by removing old solder to planarize the surface before re-balling the die. We can re-size pads as needed for partial wafer bumping.
Figure 1 shows resized pads placed over the original pads:
PROCESSES
Let’s begin with traditional aluminum pads of 50um to 100um, or with redistribution pads as large as a few hundred microns. At this scale, most metals don’t adhere to aluminum pads without conversation to a more solderable metal. Our usual choice is electroless nickel-gold (ENIG). ENIG is a quick, maskless process, saving the cost of mask design, layout and procurement. The nickel is solderable and the gold keeps the nickel from oxidizing. We also have nickel-palladium-gold for high temperature bumping
CVI first removes aluminum oxides with BPS100. BPS is better than the traditional PAN (phosphoric, acetic, and nitric mix) etch for aluminum. BPS removes the oxide without attacking the base metal. This avoids removing too much metal, which would weaken the nickel-gold and the solder joint.
Once the pads have been converted from aluminum to NiAu (typically 2um of Ni and 0.05um of Au) the desired solder alloy can be deposited. The NiAu metallization allows CVI to select any number of leaded or lead free solders. CVI has bumped pads as small as 25um and as large as 750um.
Figure 2 shows part of a die after single-chip bumping on NiAu:
CVI also has a similar process for copper pads. First we remove the oxides with BPS170, which (like BPS100) removes oxides but not the base metals. Next we deposit a layer of NiAu. As with aluminum, once the NiAu layer is deposited on copper, a variety of solders are available, ranging from low temperature indium alloys to high lead solder for server and high temperature applications.
In some applications such as RFID, where a gold stud bump is desired, we have added 20-30um Ni followed by 0.05/0.1um Au to eliminate the time, cost and variability in bump height of gold stud bumps. The NiAu pillar is suitable for RFID assembly with anisotropically conductive adhesives and films.
In summary, CVI provides a wide range of options to you for bumping single die and partial wafers, with significant savings in time and money.
FOR MORE INFORMATION
TQCollier@covinc.com
www.covinc.com
972-664-1568