1-1 Introduction to the wire bonding
Wire bonding is a process that makes the connection between an IC chip and the base material.
The leadframe and substrate are the two types of commonly used base materials. Bonding the wires onto the IC chip is a very critical procedure because the original connection areas of an IC chip are too small to be welded onto the PCB. Some base material is devised and used as the connecting medium between the IC chip and the PCB. Each of the connections on an IC chip is called a pad.
The interval between two adjacent pads is generally referred to as a pitch. The internal connector on the base material is called a lead. Typically, a gold wire of high purity (99.99%) is used to connect the pad and corresponding lead. A bonding ball is formed on the pad, while a bonding stitch is formed on the lead. One lead may consist of more than one bonding stitch. The image of an entire chip with 216 leads and 312 surrounding wires is shown in Figure 1. The image of a single leadframe unit without the IC chip is shown in Figure 2. A part of the enlarged wire-bonded IC image is illustrated in Figure 3.
The entire set up flow for the bonding machine with the correct wire bonding positions in ordinary IC packaging foundries is composed of 5 parts, as illustrated in Figure 4, and is described below. The R&D department first generates the CAD drawing (Step 1 of Figure 4) which includes the base material layout and the bonding wires with the wire bonding positions according to the required spec, as shown in Figure 5(a). The base material factory will then fabricate the base material according the CAD drawing.
When IC packaging foundries receive the base material, the production engineers set up the first bonding machine with the correct wire bonding positions based on the given CAD drawing (Step 2 of Figure 4). In setting up the bonding machine, engineers must perform the following operations and determine the associated parameters: set up the calibration marks, the initial bonding
Figure 1. Diagram of IC chips with bonding wires on the base material (leadframe).
Figure 2. Image of a single leadframe unit before the die is attached.
Figure 3. Enlarged image an illustration of a wire-bonded IC.
Figure 4. General wire bonding position setup flow in IC packaging foundries.
Figure 5. (a) Diagram of a wire-bonded IC. (b) An enlarged image of a part of (a). (c), (d), (e), and (f).
are enlarged images of some bonding samples.
positions, the coordinate values of the ending point of each wire, bonding sequence of each wire, and determine the bonding parameters (such as lighting condition). The bonding positions of each wire can be set up manually or downloaded from a CAD drawing followed by converting the designed wire bonding information into the bonding program of a wire bonder [1]. Ideally, the CAD drawing download process will correctly set up the wire bonder’s bonding program. However, in practice, some bias between the CAD drawing and the base material such as “the gap of mask design” and “process variation in base material manufacturing” (Appendix A) may still exist and the set-up must be manually calibrated. Another bias is the CAD drawing is designed for easy to read, clearly be distinguished and to prevent mis-assessment due to overlap when a lead has multi-layered wires. There are gaps between the ending points of the CAD drawing and the axial of the lead, as show in Figure 6. Therefore, in the end, after the CAD drawing download process still needs manually calibrated.
Figure 6. The ending points of the multi-layer wires are shift away the axial of the lead.
After the set up, engineers will bond a sample and check the correctness of the wire bonding positions manually according to the CAD drawing with the aid of a microscope. Once a sample bonding position is found not correct, the engineer must adjust the setting, bond a new sample and repeat the checking procedure again. After the engineers have confirmed that the wire bonding positions are correct, all setting operations and parameters from the first setup machine must be
stored as the standard bonding program. The standard bonding program is then used the next time or duplicated to other bonding machines to save the setup time.
To finish each production order, IC packaging foundries need several bonding machines to work at the same time to reduce the production cycle time. The engineer duplicates the bonding program from the first setup machine into other bonding machines (Step 3 of Figure 4). In duplicating the bonding program, engineers must calibrate the bias of the wire bonding positions, for each of the other different bonding machines, such as the initial bonding positions, calibration marks, and lighting condition. After the calibration, engineers will have the set-up machine bond a sample and execute the same checking procedure as shown in the Step 2 of Figure 4 to avoid mishandling the bias calibration. After these operations, the engineers will release this bonding machine for mass production (Step 4 of Figure 4).
Even in mass production, the wire bonding positions must frequently be adjusted manually (Step 5 of Figure 4) throughout the entire bonding process. A number of products would experience variation issues during wire bonding process. The major variations include the variation from machine to machine, platen issue, lead position variation, bad or contaminated electroplating, and thermal-induced deformation or shrinkage (Appendix B). It would require continuous adjustment of wire bonding locations. But during adjustment, it is possible to result in the wrong wire bonding due to negligence. To prevent incorrect wire bonding, it is necessary to bond a new sample and check in detail after every adjustment.
Steps 2, 3 and 5 in Figure 4 are the critical procedures in setting up the wire bonding positions and might cause incorrect wire bonding. Because ICs have become increasingly more complicated, the number of wires has increased accordingly, reaching to several hundreds. For example, the number of wires in a leadframe IC can range up to 300. It takes about 2 to 3 hours for an engineer to set up the corresponding wire positions to be bonded. During the tedious wire bonding positions
setup process, it is inevitable that mistakes will be made. Incorrect wire-bonding will occur as shown in Figure 5(d).
Because wrong wire bonding is non-reworkable – cannot be repaired into correct bonding, it is a serious cost problem in the wire bonding process. Up to the present, IC packaging foundries still rely on humans to check the correctness of wire bonding positions with the aid of microscopes.
Such manual inspection is prone to errors and cannot work synchronously with the bonding machines. A novel CAD-based vision approach is proposed to prevent wrong wire bonding in this dissertation. The proposed approach can be used to work synchronously with the bonding machine.
1-2 Objectives
The major objective of this dissertation is to develop an incorrect wire bonding prevention system, including the hardware and software. The hardware of the proposed system includes the illumination module, image capturing module and loading module, while the software of the proposed system includes the inspection algorithms and the hardware controlling module. This dissertation focuses on the control system and the inspection algorithm.
There are three focal points in the inspection system: (a) inspect the correctness of the wire bonding position on a multi-layered wire IC, (b) fully solve the problems of mal-detection (false positive, the wire is bonded in the correct position but is recognized as an incorrect bond) and lost detection (false negative, the wire is bonded onto the wrong lead but is recognized as a correct bonding case) that may occur in other available methods, (c) work synchronously with the wire bonding process in the mass production environment. The first focus of the proposed vision inspection system is to inspect the correctness of the wire bonding position on a multi-layered wire IC. To a multi-layer wire IC, such as that shown in Figures 7 and 8, the image of the wires in the lower layer would be hidden by the shade of the wires in the upper layer. The wires in the lower
layer cannot be observed and identified correctly by using the available 2D inspection methods. We developed a method which integrates image processing and the wire bonding position simulation technique, that can auto-inspect such correctness was first proposed. The proposed method captured the image of leadframe IC and checked the correctness of the wire bonding position before the actual bonding process was executed. There is no need to extract the wire from the complex varying background after the bonding process. Post-wire-bonding image identification is not required because such an inspection approach is based only on the information of the bonding sequence, the coordinate values of both end points of each wire and the base material image. In other words, there is no need to concern about whether the image of the wires in the lower layer will be blocked or hidden by the shadow of the wires in the upper layers. Therefore, the proposed approach can be used for inspecting the bonding position of multi-layer wire ICs.
Figure 7. Multi-layered wire bonding (Advanced Packaging magazine, October 2005.).
Figure 8. Enlarged image of multi-layered wire bonding (From http://www.kns.com).
The second focus of the proposed vision inspection system is to solve the mal-detection and lost detection problems that may occur in other available methods. Compared with the other substrate-based IC product material fabricated using a mask, the leadframe-based material is fabricated by punching or etching and has the problem of high variation and low accuracy for each lead. This implies that in the punching or etching process a lead will invariably be shifted and the engineer must adjust the wire bonding positions from time to time in order to fit the variation.
Figure 5(e) shows a wire bonded to a position that is slightly higher than the designed position.
However, it is still bonded on the same lead and is accepted as a correct bond. For other available methods, this wire will be recognized as being incorrectly bonded and a mal-detection will occur.
The mal-detection problem for the “Ground Bond” wire is the most critical because the allowable shift in the ground bond is as large as the entire ground bond area. Figure 5(f) shows a ground bond that is shifted slightly to the right but is still in the acceptable position. Another issue, electroplating quality, will also affect the leadframe quality and consequently affect the wire bonding positions.
To filter out the mal-detection or lost detection problem in the bonding position inspection, a set of associated inspection algorithms were also proposed.
The third focus of the proposed vision inspection system is to have it work synchronously with the wire bonding process in the mass production environment. In mass production environment, the wire bonding positions must frequently be adjusted manually throughout the entire bonding process.
To prevent incorrect wire bonding, it is necessary to bond a new sample and check in detail after every adjustment. The proposed vision inspection system must fast enough to work synchronously with the wire bonding process.
By using the proposed image process and inspection algorithms, we can develop an AOI system to detect and prevent all the incorrectness wire bonding.
1-3 Scope/limitation of the dissertation
The proposed vision inspection system is focused on the leadframe-based material IC product.
The substrate-based material IC product or other special based material IC product is not included in the study.
The defects that may occur on the bonding wire can be classified into the problems of incorrect bonding position and wrong shape of the bonding wires. The proposed vision inspection system is focused on the defect inspection of incorrect bonding position. The inspection of the wrong shapes of the bonding wires is to detect whether any wire is shifted, shorted, or sagged during the wire bonding process. It still need to rely on post-wire-bonding inspection and is not included in this study.
1-4 Organization of the dissertation
This dissertation is organized as follows. Section 2 presents a survey of the researches on wire bonding position inspection. The proposed system is described in Section 3 and Section 4. Some inspection experiments and the performance analysis of the proposed approach are presented in Section 5. Section 6 presents the conclusions and future researches.