In advanced packaging, the increased attention on flip chip (FC) and chip scale packages (CSP) – fueled by the need to lower signal path transmission delays and reduce circuit board real estate – is only part of the story. How precisely these packages are placed now poses a dramatic impact on the process owner`s bottom line. As flip chip pitches move down to 150 microns and lower, and the use of non-self-aligning processes grows, many current surface mount technology (SMT) types of machines will be eliminated from handling these accuracy-dependent processes.

 

To be sure, as semiconductors become more complex and their packages become smaller, the accuracy of the machines placing these semiconductor die into packages or substrates is key. It has become imperative for manufacturers to understand accuracy fully, and that includes how a machine`s ability to maintain accuracy will affect yields in the long term. In addition, end users will be challenged enough in the future just dealing with semiconductor production yields because of die size and complexity. They should not have to worry about a die placement machine adding to yield losses due to misplacements.

 

Defining the Terms

 

Inconsistencies occur because accuracies stated by some equipment suppliers are really “sales” accuracies. They are merely stated to generate sales and are not achievable in a production environment. “Real accuracy” is measured by a machine`s performance a year or two after installation, after having performed 5 million to 10 million pick-and-place cycles. “Placement accuracy” is defined, in general terms, as how close the placement came to where it was supposed to be. Thus, it is the difference between the actual placement location and the desired placement location. For a pick-and-place machine, which uses encoder feedback to determine the proper location, this translates to how well it can move to a location by calculating the encoder counts it takes to arrive there. This is opposed to the term “repeatability” which, for a machine, is defined as how well it returns to a taught (known encoder count) location. Therefore, any machine should be far more repeatable than accurate.

 

Accuracy comes into play when a machine is programmed to move 100 mm from its current location. Or, as with most equipment today, it uses a vision system to generate offsets to known data for a placement location. Accuracy, as defined above, is the most critical parameter for pick-and-place equipment. Quantifying placement accuracy becomes even more critical. This is where statistics come in and why machine specifications should use terms like “±25 µm 3s.” If specifications are not described this way, buyers should be aware that the accuracy of the machine is not really known or is not a selling point. Without a statistics course, this accuracy nomenclature can be difficult to understand.

 

After a machine picks a part, uses vision to determine the offsets and places the part, accuracy is then measured and this process is repeated many times. The machine does not always return to the same location, but will form a random pattern around the ideal placement location. Most placements will be very near ideal and a few could be quite far (in relative terms) from it. When this data is charted to count the number of placements within concentric rings around the ideal placement location (Figure 1), a histogram graph results (Figure 2). If a line is drawn through the tops of each of the histogram bars, a shape known as a bell curve (Figure 3) results (also known in statistics as a normal graph). From this data, standard deviation can be calculated; this indicates how tightly bunched the locations are around an average location. The standard deviation is also known as sigma (s). Thus, a standard deviation of 10 µm (the same as saying ±10 µm 1s) would be a narrow bell curve (and a better accuracy number) than a standard deviation of 50 µm (a much wider bell curve).

 

The standard deviation number is linear when extrapolating out. Thus, ±10 µm 1s is the same as ±20 µm 2s and the same as ±30 µm 3s. This also means about 68 percent of placements are within ±10 µm, about 96 percent are within ±20 µm, and about 99 percent are within ±30 µm. If a machine has stated ±30 µm @ 3s accuracy, then a majority of its placements should be within ±10 µm. If a manufacturer only states an accuracy of ±10 µm without a sigma number, it can be assumed the reference is to 1 sigma at best (or, worst case, it may mean at least one part has been placed in a lab within those specifications).

 

Going Smaller, Faster

 

Two words sum up why it`s so important to understand a placement machine`s accuracy: Smaller, faster. Flip chip bumps are getting smaller to allow for either more I/O per die or for smaller die. Many current bump diameters are down to 75 to 80 µm at 150 µm pitch. New packaging techniques, such as silico