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26 March 2014

Inexpensive In-process Planarity Adjustment for Die Attach

Posted in Application Notes

Equipment: Model 860 Eagle Bonder

 

Application: Inexpensive In-process Planarity Adjustment for Die Attach
Equipment: Model 860 Eagle Bonder

 

 

Process: In many cases it is critical that the surface of a die and substrate be coplanar during the bonding process. Planarity is fairly easy to control in the case of die that are a few millimeters or less in their largest dimension. Squared up stages and die tools will normally do the job. Die with a dimension in the ten millimeter range may require individual adjustment of each die to its’ mating substrate. The main issue is the substrates are not true enough and the dimensions are not held closely enough from part to part to achieve acceptable planarity. Typical examples of applications that need in-process adjustment are laser bars and focal plane arrays. Laser bars are generally about one millimeter wide by ten millimeters long. It is in the long axis that planarity is most critical. Focal plane arrays are generally about ten millimeters square. In this case, both the length and width axis is critical. The typical substrate for these examples will be flat to about plus or minus five microns and worse. The planarity required is about plus or minus one micron. Clearly, the planarity will need to be adjusted for each set of die and substrate..

 

Solutions:

There are several methods that can be used to adjust the planarity between a die and substrate. I will discuss three of them in this paper.

First is laser interferometer and laser scanning. I put these in the same category because they both use a laser to measure the plane of the substrate and then make automatic adjustments to eliminate the error. This method is accurate but expensive. Drawbacks can be the reflectivity of the substrate, irregular surface of the substrate, and the high price. By its nature, this method is only found on very expensive equipment, which falls outside the “economic” category.

Second is mechanical self-alignment by use of a flexible element or a slipping or pivoting joint. It is possible to achieve planarity between two surfaces by holding the surfaces together and allowing them to conform to each other. One of the two parts involved could be held by a tool with a flexible shaft. As the surfaces are pressed together, the shaft bends and the surfaces match planarity. This method can be inexpensive. An undesirable side effect is the alignment of the die and substrate will be undermined as the tool bends. This makes flexible or compliant tools unacceptable for most of these applications.

Another method of mechanical self-alignment is to design a pivot into the die tool. The die can be brought into alignment with the substrate by bringing them into contact while the pivot is free to move. After alignment the pivot can be locked and the die and substrate can be separated for final alignment and then brought back together. This method has the disadvantage of causing a sliding movement between the die and substrate. The pivot point by the nature of the design will be on a different plane than the die/substrate interface so the tool will move in an arc as it pivots, which leads to the sliding movement. Another disadvantage is this method can only work in one axis with any practical design. Long narrow die such as laser bars will work here because planarity correction is only needed in one axis. This will not work with large square die such as focal plane arrays. These die need correction in two axes and the sliding motion would likely cause damage to delicate bumped die.

A third method is tactile alignment. This is basically a mechanical process that employs load sensing with a goniometer stage. The load sensing system monitors and displays the load in grams with a five-gram load change roughly equal to one micron. The goniometer stage tilts the substrate with the pivot point on the same plane as the die/substrate interface. With the die in light contact with the substrate, the substrate is tilted back and forth slightly. The load will increase as planarity is worsened. The point where planarity is best will be the point where the load is lowest. The goniometer is simply set where the load is lowest for planarity within about one micron. After planarity is set the alignment of die and substrate can be confirmed and the die bond process completed.

 

Model 860 Eagle Bonder: Semiconductor Equipment Corporation can provide an optional goniometer stage with one or two adjustable axes. When combined with our strain gauge load sensor, which is built into the bond head, the Model 860 has the ability to adjust planarity for each set of die and substrates as part of the bonding cycle. This tactile alignment approach is easy for the operator to understand and apply. Planarity within one or two microns is readily achieved.

 

Contact: Lester Salvatierra
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