By Lau Siu Wing and Joseph Poh, ASM Assembly Automation Ltd., Hong Kong
Many of today's chip-scale/chip-size packages, particularly ball grid arrays, share the attributes of conventional leadframe ICs. This similarity often enables manufacturing floors to take advantage of common machine platforms to address die attach requirements. However, experience has shown that there are still areas of concern that assemblers must address to avoid costly yield and potentially damaging reliability problems.
Substrate Considerations To achieve consistency during production runs for such attributes as die placement, die rotation and bondline thickness, the die bonder's design must be able to address the geometrical accuracy of the substrate. It must also be capable of handling the non-metallic nature of the material. A typical wire-bonded CSP (See Figures 1 and 2) will use BT resin glass as the substrate material with a dimensional tolerance of ± 0.15 mm in length and ± 0.075 mm in width and an accumulative indexing hole tolerance of ± 0.1 mm. This excessive (when compared to typical QFP leadframes) substrate tolerance will further affect the accuracy of the overall die placement on the specified position. A likely result will be placement of the die too close to the lead, which may cause the epoxy to short the leads. To avoid this result, the die bonder should be able to "zero" the tolerance of the given position of each individual Die Attach Pad (DAP) area. This alignment capability should cover an effective die bonding area of 50 mm in the X direction and 65 mm in the Y direction. To meet the above requirements, a die-attach machine with a built-in pattern recognition system that can feedback the tolerance of each individual unit to the bond head is required. A high-resolution (1.2µm) XY movable bond head will use this information from the alignment points (as shown in Figure 3) to compensate for any inaccuracy prior to die attachment. This results in a typical overall die placement accuracy of 10µm at 1 sigma.
Bondline Thickness Providing a consistent epoxy spread for low voids and bondline thickness control is vital to the manufacture of BGA packages, because the space between the edge of the die to the lead is typically less than 0.3 mm (12 mils), while the thickness of the overall package is commonly 1.56 mm. The result is a typical bond line thickness requirement of less than 0.025 mm (1mil), as illustrated in Figure 4.
To provide a desired pattern with a consistent epoxy volume to each package unit, volumetric epoxy writing systems are employed. These systems offer users the flexibility of either creating their own epoxy pattern or simply relying on the machine's built-in patterns to perfectly fit the require die size. (Examples of epoxy patterns available are shown in Figure 5.)
The XY table of the epoxy writer offers an accuracy of 4µm at 1 sigma and 3µm at 1 sigma for the Z-axis. The high accuracy of both XY and Z repeatability guarantee achieving the required shape and dimension of the epoxy pattern.
After writing the consistent epoxy pattern on the die-attach pad, it is the bondhead's responsibility to place the die consistently at the same level on every individual unit. Achieving the required bondline thickness consistently calls for a consistent DAP surface, a highly repeatable bondhead Z travel with reasonable resolution, and a set of process-controlled parameters. Figure 6 shows the cross section of a die-bonded unit using a bond force of 50 grams and a bond time of 40 milliseconds. Based on a measurement sample size of 30 units, the mean bondline thickness achieved was 20.56µm with a standard deviation of 1.71µm. Process Monitoring BGA packages are usually classified as higher cost products, compared to leadframe packages offering the same electrical functions. It is, therefore, necessary to instantaneously monitor the status of both pre-bond and post-bond die attach conditions to ensure production yields. Typical inspection items for prebond are epoxy position, epoxy shape and the amount of epoxy on the lead, as shown in Figure 7. Typical post-bonding items are die placement, die rotation, epoxy coverage and epoxy on die, as illustrated in Figure 8.
The ability to detect the epoxy condition on a dark green substrate is completely different from detecting conventional epoxy on a leadframe DAP, where the metallic surface has a much higher reflectivity index than the epoxy. Use of a special LED lighting system, as well as an intelligent software algorithm to overcome reflection similarities between the substrate and epoxy is needed. If a statistical record on the die placement or die rotation is required, the information can be obtained from the machine, or it may be printed out via a RS232 protocol. A typical display of the results for die placement is shown in Figure 9.
Skip Bonding on Defective Units Besides the above machine placement requirements, typical CSP substrates require the machine to be capable of foregoing dispensing and bonding of good die on defective units. Obviously, the location of the defective units, such as epoxy on die or a substrate problem for each individual strip of substrate, is very useful for subsequent downstream processing. Moreover, the creation of a strip map by the die attach machine, when it captures all known substrate/die-attach defects and combines them with unique laser marking of each strip with 2D matrix codes, allows for subsequent operations to use this data automatically.
Conclusion Driven by the requirements for package miniaturization and reliability, CSPs have become a major packaging technology in today's microelectronics industry. The similarity in the assembly process between a BGA package and a leadframe package enables the manufacturing floor to exploit the commonality of machine platforms to address sophisticated die-attach requirements. Despite process similarities, however, there are still many areas in die bonding BGAs that must be addressed with next-generation technology to avoid costly yield and reliability problems.
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