Due to the increased integration availability of bumping connection,
the Flipchip technology has become a popular assembly technique.
The assembly of Flipchip die presents a few challenges depending on
several factors including the position, size and shape of the bumps.
The selection of the adequate tools usually allows a range of diverse
solutions and sometimes would require a custom tooling approach.
In the following examples, we will show some of our recommendations.
|Basically, the placement of a flipchip is made in 4 steps.|
|Step 1 - Pick-up the die from the bumped surface||Step 2 - Rotate the die on flip
|Step 3 - Pick-up the die from the back side||Step 4 - Place the die on the
To perform the complete cycle, two vacuum pick-up tools are necessary. Those are to be selected based on the bumps quantity, position and based on the die attach process during placement.
|Step 1 - Pick-up the die from the bumped surface|
|A) The best method is to avoid touching the bumps. In many cases, it will be possible to use a round or rectangular peripheral (3,4,5) surface tool with appropriate cavity or even a die collet (10) and making contact only with a flat surface around the bumps orwith the die edges. Depending on the position of bumps, RT 4xfeet (6,7,8) tool or tool like a CT (9) could have access to unobstructed areas.|
|B) The alternative is to make physical contact with the bumps. The main problem is vacuum loss leading to missing pick-up or die misplacement due to unstable process. Softer materials have proven to provide a better vacuum force than harder material. The second concern is displacement or removal of bumps generated by the impact of tool over the bumps. Here also, a soft material is preferred but the expected tool lifetime will be reduced.|
|Bump contact tool selection
The first idea is to make use of a traditional soft rubber or thermoplastic elastomer. Since many years, the FCTR (11-12-13-14) replaceable tip has found satisfaction where a gentle touch of the tool lips was required. The diameter of the tip is chosen slightly smaller than the area covered by the bumps.
|An alternative to the standard pick-up face geometry is to use a rubber or thermoplastic elastomer pad with multi-hole grid similar to our FRTR-MH (15,16,17,18) product. Here also, the material must be selected to avoid bump damage. As with previous solution, we have to take into account a vacuum force drop at contact surface due to leaks.|
|In certain applications, when the bumps are relatively large and few, it is possible to build a tool having vacuum holes exactly corresponding to the bump position on the die. The contact is still made with the bump but only on the top while with this solution, the vacuum leak is eliminated. The material can therefore be harder than previous example. Figures 19, 20, 21.|
|There will still remain applications where none of the above techniques can be used, supposedly because of insufficient vacuum to guarantee a precise pick-up and placement process. In this case, another approach is necessary. One solution is to use a tool with adhesive having a balanced tacky force allowing bumped die pick-up and later to allow release of die after flipping for back-side die pickup.
Figures 22, 23, 24.
Step 3 - Pick-up the die from the back side &
Step 4 - Place the die on the substrate.
|Once the chip has been flipped over, we have a large and unobstructed surface available for the pick-up process. At this stage, it is the bump connection to substrate which becomes the main factor for tool selection. One example is when the bumps are made as stud bumps with gold wire on traditional wire bonders and attached to the substrate by thermo-sonic process.
In this case, the tool shape and length has to be adjusted to the ultrasonic system in order to generate the desired tip vibration and amplification. The die collet (25,26,27) has an optimum design to guaranty a good energy transfer by better securing the die inside its internal pyramid walls during tip vibration.
|The easiest configuration, for a die placement tool, is when the bumps are connected through wetting of epoxy paste. Small errors in parallelism will be allowed and therefore, most of the surface pick-up tools can be used like plastic RT (28,29,30) tool or PCTR, BPCT, CTR (31,32,33) etc. The only restriction may be related to the tool tip temperature resistance if the process uses heat energy during die placement.|
|With thermo-compression, the force must be distributed over the entire back die surface. The material must have a good thermal conductivity. For such applications, a tungsten carbide RT (34,35,36) tip with a relatively small vacuum hole can be selected.|
|There are situations where the parallelism between the tool and the bumps level must be optimal. This is made possible by compliant tools (36 to 42) which will adapt the parallelism through a flexible connection between the tool tip and the holder. Several designs are possible, we show here a few examples.|