|
 |
The merchant semiconductor packaging market has grown dramatically throughout the 21st century, as the world’s chip producers collaborate with outside specialists to minimize time to market and design costs. Proving the robust health of this market, the global market leader, Amkor, posted record sales for the first quarter of 2006, up 55% on the same period in 2005. The market is growing, as are the number of businesses serving it, as packaging performance now influences many advanced designs.
Today’s increasingly competitive packaging specialists are driving demand for processes and technologies that are both fast and flexible. As an example, most specialists now operate a stencil printing – or mass imaging - process to bump at die and substrate levels, and to deposit other electronic materials including fluxes and conductive adhesives. These have delivered a number of advantages including high throughput, high equipment utilization, low capital costs, and small physical footprint
Loading and Unloading for Speed and Yield
One of the fundamental challenges to implementing an effective mass imaging process lies in delivering the work-piece, which may be a wafer or singulated substrate, into the mass imaging machine, and to establish and maintain correct alignment throughout deposition of the flux, solder balls or other material. To maximize throughput and minimize damage to individual pieces, and thereby drive up yield, A number of solutions have emerged that allow singulated substrates, for example, to be processed directly from the process carrier. This is usually a NEMI or JEDEC standard such as an Auer boat. This is desirable since unloading and reloading each substrate individually incurs substantial time overhead, and also raises the potential for damage to the substrates.
“Virtual Panel” solution
To maximize throughput, which is the main advantage of mass imaging in semiconductor packaging, some solutions have emerged that allow all the substrates in the process carrier to be printed in a single cycle. The principle is to align each singulated substrate individually in x, y and theta axes before the squeegee begins its excursion, and also to raise all substrates to the same height, creating what has been termed a “virtual panel”. The squeegee then passes over the stencil, printing the deposit onto each substrate.
This Virtual Panel Tooling has proved effective where large numbers of substrates are required to be imaged quickly, particularly when underfilling, encapsulating or printing solder to attach SMT passives. On the other hand, some limitations include the inability to prevent printing onto a known defective substrate. Each unit in the virtual panel is imaged.
Resolution for fine pitch imaging is also restricted. This is because a practical virtual panel tooling arrangement will typically align the substrates mechanically, with reference to two corners. Such a mechanism achieves adequate accuracy for many substrate processes. However, in some processes where repeatability is critical, such as solder ball attachment, a virtual panel solution may not be optimal. In addition, component designs requiring attachment of SMT passives are now seeking to use small outline 01005 components instead of the larger 0201 or 0402 form factors. Hence it can be seen that accuracy and repeatability requirements for substrate level processes are on the rise.
Individual Processing
To achieve greater precision when imaging multiple substrates, alternative tooling solutions that present singulated substrates to the process individually have emerged. Upon arrival of the carrier loaded with singulated substrates, each substrate is raised in turn from the process carrier and aligned with the stencil before printing. While this naturally trades off some of the throughout advantages of printing materials for substrate-based assembly, there are a number of benefits. For one, if the co-ordinates 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.
In addition, it becomes feasible to use the machine vision system to align the substrate. This results 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, which requires extremely high repeatability. Since the machine vision system is included as standard on advanced printing platforms used for semiconductor assembly, this capability comes at no additional cost in terms of extra performance options that must be fitted.
Some of the issues to address in the design of a single-substrate tooling solution include provision 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, which is known to have a beneficial effect on paste volume repeatability. 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, by including a surround plate as part of the tooling solution, to have a similar effect to that of the virtual panel surround plate, the process becomes essentially independent of the width of the squeegee. Hence, a larger squeegee can be used, which allows greater control over squeegee pressure; leading to improved process capability. Furthermore, 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 for greater utilization and productivity.
Another important factor is that, when performing alignment using machine vision alone, the time required to align the substrate can be relatively long. Process carriers are not designed as precision instruments, with the result that there is significant opportunity for substrates to move in all directions while in the carrier. The alignment mechanism therefore 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 pre-alignment stage may be implemented cost-effectively using a mechanical solution similar to that employed in the virtual panel tooling solutions mentioned earlier. This can be implemented at low cost 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. The effect of this centralization stage is to significantly reduce the stencil movement required as the machine vision camera begins to calculate the co-ordinates for perfect alignment. The excursion time, therefore, is in turn reduced.
Of course, using the 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, for example. More importantly, if fiducials are provided on the substrate, even greater accuracy and repeatability can be achieved. Arguably, this could enable alignment to wafer-level accuracy. 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 micron could be achieved using the latest singulated substrate alignment techniques.
Solder Ball Placement on BGA Substrate
As an example, solder ball attachment to substrates during ball grid array package assembly is set to benefit from the increased accuracy and repeatability, and faster alignment, that can be achieved using the latest 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 on 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. Accuracy and repeatability are of paramount importance. Errors in stencil alignment will produce defects such as smearing on the stencil underside, as well as unattached solder balls or poor stencil release leading to removal of solder balls upon separation. The results of these unwanted effects will include poor first pass yield and excessive cleaning, leading to low assembly quality and productivity.
Accurate alignment, driven by machine vision, therefore, is essential for 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, the precision alignment solutions now emerging are also suitable for pre-packaged, 3D components. Combined with the versatility of the mass imaging platform, a wide range of high resolution and fine pitch packaging processes can be supported, significantly lowering cost of ownership for merchant packaging specialists, as well as OEM packaging groups. Moreover, high accuracy processes can be hosted on a standard printing platform, thereby enabling advanced packaging processes to be implemented at low cost, with no loss of flexibility or throughput.
In addition, SMT assemblers are able to leverage existing investment in automated, in-line printing platforms to offer a subset of back-end packaging services in addition to standard SMT. Tooling for individual processing of singulated substrates is directly compatible with the standard screen printer tooling bed, and can therefore be used on a completely standard printer platform. Another benefit is that this can be also changed over quickly and efficiently to perform packaging or standard SMT processes.
|
|