Substrate Handling for High-accuracy Packaging Processes
09/01/2006
BY STEVE WATKIN, DEK
Today’s packaging specialists are driving demand for processes and technologies that are both fast and flexible. For example, many specialists operate a mass-imaging process to bump at die and substrate levels, and deposit electronic materials.
One challenge to implementing an effective mass-imaging process lies in delivering the wafer or singulated substrate into the mass-imaging machine to establish and maintain correct alignment throughout deposition of the flux, solder balls, or other material. Singulated substrates can then be processed directly from the process carrier in a single cycle. This is a desirable alternative to the time-consuming unloading and reloading of each substrate individually, which increases the potential for substrate damage. Each singulated substrate is individually aligned in x, y, and theta axes before the squeegee begins its excursion, and all substrates are elevated to the same height, creating a “virtual panel.” The squeegee then passes over the stencil, printing the deposit onto each substrate.
 Figure 1. During the machine cycle, the second substrate is in a position to be lifted out of the carrier by the tooling tower. |
Virtual panel tooling is effective when many substrates need to be imaged quickly, particularly when underfilling, encapsulating, or printing solder. However, it is impossible to prevent printing onto a known defective substrate because each unit in the virtual panel is imaged. Resolution for fine-pitch imaging is also restricted because a practical virtual panel tooling arrangement will align the substrates mechanically, with reference to two corners. Such a mechanism achieves adequate accuracy for many substrate processes, but when repeatability is critical, such as during solder ball attachment, a virtual panel solution may not be optimal. In addition, using small outline 01005 components instead of the larger 0201 or 0402 form factors is preferred for component designs requiring attachment of SMT passives.
Individual Processing
To achieve greater precision when imaging multiple substrates, alternative tooling solutions have emerged. When a carrier loaded with singulated substrates arrives, each one is raised in turn from the process carrier and aligned with the stencil before printing. If the coordinates of known defective units have been recorded, it is possible to direct the sequence to pass over those units and raise only those that are not known to have a defect. This can be performed automatically by querying the data for the carrier, which can be updated and stored in an online database. It is also feasible to use the machine vision system to align the substrate, resulting in a more precise alignment, capable of achieving repeatable results when performing high-accuracy imaging required for 01005 passives or for fine-pitch solder ball attach.
The design of a single-substrate tooling solution requires provisions of support for the stencil in the area around the raised substrate. In a virtual panel tooling solution, by contrast, the substrates are all raised to be coplanar with a surround plate that extends between the printer rails. This provides adequate support across the full width of the squeegee.
 Figure 2. In virtual panel tooling, the substrates are all raised to be coplanar with a surround plate that extends between the printer rails. |
It is possible to achieve high repeatability when processing substrates individually using a small squeegee equal to the width of the substrate. On the other hand, including a surround plate as part of the tooling solution allows the process to be independent of the width of the squeegee. Therefore, a larger squeegee can be used, allowing for greater control over squeegee pressure and leading to improved process capability. Devices in various sizes can be processed without frequent squeegee changeovers, and a larger volume of print medium can be dispensed onto the stencil, leading to longer replenishment intervals.
Performing alignment using only the machine vision can be a timely process. Process carriers are not designed as precision instruments, so significant opportunities arise for substrates to move in all directions while in the carrier. Therefore, the alignment mechanism may be required to perform a large excursion to fully align the substrate. One solution is to arrange a pre-alignment step in between raising the substrate and activating the machine vision camera to perform the final, high-accuracy alignment. This can be achieved using alignment pins located at two corners - for example, the upper right and lower left corners - to quickly perform a centralizing action as the substrate is raised from the process carrier. This centralization stage reduces required stencil movement as the machine vision camera begins to calculate the co-ordinates for perfect alignment, shortening the excursion time.
Using a machine vision system also opens the possibility of using alternative reference points on the substrate to perform a more accurate alignment than is possible using a mechanical, edge-referenced scheme. Optical detection of the substrate edges allows the substrate and stencil to be aligned closely enough to support a repeatable solder ball placement process. More importantly, if fiducials are provided on the substrate, even greater accuracy and repeatability can be achieved. With increased accuracy and repeatability comes the potential to address even finer dimensions. The limits of current machine vision technology indicate that substrate bumping with interconnect pitch as fine as 200 µm could be achieved using singulated substrate alignment techniques.
Solder Ball Placement on BGA Substrate
Solder ball attachment to substrates during ball grid array (BGA) package assembly will benefit from increased accuracy, repeatability, and faster alignment achieved using single-substrate imaging solutions. The process involves an initial flux deposition stage, which requires accurate alignment of the fluxing stencil with the solder ball sites. These may be positioned on a full grid array or a perimeter grid array. Flux is applied to each substrate in turn. When the last substrate is placed in the carrier, all are fluxed and ready to move to the solder ball placement stage. Each substrate must then be individually re-aligned with the solder ball stencil to place solder balls in precisely the same location as the flux. Errors in stencil alignment will produce defects such as smearing on the stencil underside, as well as unattached solder balls or poor stencil release that leads to removal of solder balls upon separation. The results of these unwanted effects will include poor first-pass yield and excessive cleaning.
 Figure 3. The surround plate uses an integral clamping function to “square up” substrates for printing support. |
Accurate alignment, driven by machine vision, allows packaging specialists to offer high-quality, profitable BGA assembly services and other technologies requiring solder ball attachment at substrate level. The latest tooling solutions - which perform mechanical centralization as well as camera-driven alignment - can perform a complete solder ball placement cycle for a single substrate within 20 seconds. Accurate deposition of other materials, such as flux, solder paste, or conductive adhesive, can be significantly faster.
Standard Platform
In addition to solving the challenges associated with singulated substrates, precision alignment solutions are also suitable for pre-packaged, 3-D components. Combined with the versatility of the mass-imaging platform, a range of high-resolution and fine-pitch packaging processes can be supported. High-accuracy processes can be hosted on a standard printing platform, enabling advanced packaging processes to be implemented at low cost, with no loss of flexibility or throughput.
STEVE WATKIN, semiconductor and alternative applications manager, may be contacted at DEK, Granby Industrial Estate, Weymouth, Dorset DT4 9TH, UK; 44/ 1305 760760; E-mail: [email protected].