26 March 2014
Automation in Die Bonding: When Less is Better
by Don Moore, President, Semiconductor Equipment Corporation
Meptec Report, January/February 2000
|The proliferation of diverse high density package designs being developed and marketed today call for die bonding equipment that can deliver a combination of precision, versatility and speed. How much is needed of each will depend on the application and that, in turn, will determine the degree of required automation. To make the appropriate equipment decision - semiautomatic or fully automatic die bonder - when process development and follow - on low volume productions runs are involved, the responsible OEM engineer or contract assembler needs to take a hard look at the advantages and disadvantages of installing each type of system...because in many cases less automation is better.
The capability requirements for die bonding equipment that can handle over time a variety of area array package interconnections (flip chip, chip scale) can be wide ranging. One of the process engineer's biggest challenges is the different flip chip designs that keep popping up and the corresponding means that may be required in handling the attach process - e.g., solder reflow, polymer attach, eutectic, etc. Depending on design, placement precision of +/- 12 microns will suffice. With others +/- 5 microns or better may be needed. Today, there are commercially available semiautomatic die bonders that are capable of placing flip chips within +/-5µm and that feature broad flexibility in operating cycles.
When comparing the speed of a semiautomatic with that of an automatic, it is important to look beyond throughput rate and consider the complexity of set - up that will be involved. Such things as teaching a fully automatic die bonder to correlate the chip and the target site on the substrate and to handle such parameters as range of magnification and Z motion can be horrendously time consuming. For example, it is not at all uncommon for the set - up routine on automatics to take the better part of a day compared to as short a time as 15 minutes with semiautomatics. So, if you are only going to need to run a small number of chips, you can have the set - up of the semiautomatic system accomplished and the chips run in the time it would take just to get the automatic ready to run the chips.
The following presents the capability features you should look for in the basic system design, should you decide to go the semiautomatic route, as well as some of the options that will enhance operating capability when added to the platform. The designs described are all commercially available and are targeted for use in one of three types of process applications - prototyping, production start - up, low - volume production. The range of prices you should expect to pay for the designs also is given.
|Basic Requisite Features
Regardless of the high density interconnect application, the basic platform should provide some means (after die targeting and pickup) of achieving alignment of the die with the substrate site, applying bond load and adding process heat.
Alignment should be accomplished by means of a video system which uses a cube beam splitter for simultaneously viewing (in the case of flip chips) the die and substrate images. However, the video system will not allow the operator to observe the actual bonding process in real time. Therefore, it is sometimes desirable also to have a magnifying system such as a direct viewing microscope both for monitoring the chip placement process and troubleshooting (especially useful in eutectic die and laser diode attachment). The ability of the cube beam splitter in presenting the bumps on the flip chip superimposed over the pads on the substrate is critical to the alignment process when dealing with micro flip chips. Fiber optic illumination should be provided to assure proper viewing in the presence of different light levels. A video camera with motorized zoom lens for optimized viewing over a range of magnification rounds out the minimum package of features needed on the platform for achieving pick - up and alignment. Note that with such a semiautomatic system no set - up is required whereas with an automatic system the alignment process is accomplished via a pattern recognition system which, incidentally, needs to be taught.
Depending on the application, the precision that the machine has in applying bond load may or may not be critical to the success of the assembly process. When solder is used all that needs to be done after the chip has been dipped in flux is to apply just enough load (50 to 300 grams) to cause the chip to properly seat on the substrate. With epoxy and adhesive applications, both bond load and time during load become an issue. With those applications the chip needs to be held at a load of up to 10 kilograms while curing takes place in order to achieve good attachment. Other application examples where precise bond load control is required include runs with small delicate chips (which may require load control down to as low as 5 grams) and certain cases where bond loads must be different at each point in the cycle - e.g., one load at pick - up, another at flux dip and another at placement. Note that with the automatics, bond load capability is not a problem since they come equipped with excellent universal bond load control (which is something, as noted for solder applications, may not be required).
In the case of flip chip solder reflow applications, a separate hot plate or controlled atmosphere oven may be used or you may wish to accomplish reflow on the same machine that places the die. If you choose to go the latter route, the machine needs to be equipped to deliver the heat in one or more ways, depending on the application - e.g., via a heated die tool, an elevated temperature stage with a set point, a stage that can be elevated rapidly through the cycle, a stage with a very high temperature capability (500°C or more), or some kind of spot heating tool such as a hot gas heating nozzle. Note that with the latter spot heating capability you will be able to heat one chip at a time and thus do rework.
An automatic machine will provide the same levels of heating capability. However, in the case of rework it is not practical to use an automatic system.
With the bonder design selection approach being described in this article, the appropriate types of features on the basic platform covered above as well as their degree of design sophistication can be specified at the time of purchase.
Before describing some of the add - on features that are available, the application environment in which the bonding system will operate needs to be considered. This will help determine the level of capital investment needed and provide an insight into the kinds of add - ons that may be desired. For example, there are low cost semiautomatic table top starter units available that are designed for firms just getting into flip chip attachment. See Figure 1 for an example of one of these entry level die bonders. The units provide an appropriate amount of alignment precision, controllable bond load and heating capability. The design shown delivers placement accuracy of +/- 12 microns and can be used to attach flip chip, chip scale devices and bare die on circuit boards and flex circuits. This particular bonder design takes only about 15 minutes for an untrained operator to master the machine and begin running parts. It comes equipped with a waffle pack pickup stage, four - inch square vacuum stage (optional sizes available), bond load of 50 to 300 grams (2 kg option available), a manual or motorized flux tray, and has a throughput rate of 120 placement per hour.
A scanning video feature accessible via an adjustment knob on the cube beam splitter allows the operator to pan back and forth a half inch in each direction off center when working with large flip chips having various bump sizes and pitches - e.g., smaller or larger than four mills diameter and distances of less or greater than six mils. The X - Y precision slide table is equipped with pneumatic brakes. Push button - activated Z motion initiates the placement cycle. The substrate holder includes a micrometer - adjusted X, Y and theta stage.
Options that can be added to the basic platform include a heated stage capable of 350°C operation and a hot gas spot heating system (latter allows the operator to do rework). Expect to pay between $30,000.00 to $40,000.00 for such an entry level system.
Multi - use designs such as that shown in Figure 2 represent the next level up in complexity. These semiautomatic table top designs feature a combination cube beam splitter and pivoting direct viewing scope and are aimed at firms who need more system versatility for doing low volume production runs involving gold, solder, adhesive, or stud - bumped flip chips as well as laser diodes and eutectic die.
Such systems are capable of aligning and attaching die sizes usually from 0.006" to 1" sq. with +/-5µm placement accuracy at a throughout rate of up to 120 placements per hour. Standard options normally offered include a motorized rotating collet, motorized sweep (for easy perimeter viewing) and rapid ramp rate control. For extremely high temperature processing, the system can be equipped with a stage capable of ramping at 20 degrees a second up to 500°C or more. The out - the - door price for such a multi - use designed bonder ranges from $55,000.000 to $70,000.00.
|Level 3 Capability
The third level of bonding system capability (and complexity) is represented by the floor - mounted semiautomatic die bonder shown in Figure 3. This type of system is targeted for use by research labs and process development departments who need very versatile cycling capabilities and a large workstage area to carry out prototype development and characterization studies involving a host of cutting edge technologies, including laser diodes, next generation laminate circuits, etc... The bonder delivers +/-5µm placement precision of die sizes from 0.010sq. to 1'sq. and spot heating capability via hot gas. The machine features elevated stage temperature, closed loop bond load control over a range of 5 grams up to 10 kilograms (higher loads available for applications such as chip on glass) and can be equipped with ultrasonic scrub for thermosonically bonding microwave flip chip die. It provides a 6 inch sq. heated stage with 6 inch motorized travel in X, Y, and theta and a motorized zoom and pan viewing feature for scanning large chips at high magnification. The operating cycles are programmed into the machine and saved using a system of macros. The out - the - door price range for such a system is $90,000.00 to $110,000.00.
Each of the above described three levels of system capability will give the operator maximum throughput rates of around 120 units per hour. For R & D applications and most low volume (to even some mid volume) runs this will be sufficient. If the production requirement calls for higher throughput, the faster operating automatics with their pattern recognition, matching, etc..., features enter the buy decision process. While automatic die bonders will provide the required higher throughout, they basically perform the same primary steps (alignment, bond load and heating) in attaching the die as do the above three tiers of semiautomatic bonders...but the automatics will take longer to set up. Remember, in the time it takes to set up an automatic you can be finished with the chip run on a semiautomatic. And automatics cost considerably more (several hundred thousand dollars).
|As appeared in Meptec Report - January/February 2000|