Ball grid array (BGA) packages having 0.4mm ball pitch require careful attention to printed circuit board (PCB) design parameters to successfully yield reliable and robust assemblies; the standard rules of thumb don’t apply anymore. In fact, the design guidelines for 0.4mm and 0.5mm differ primarily due to issues surrounding shorts or opens between balls under the processor.
In addition to the design rules, fine-pitch board design is a team effort. Close coordination and communication between the device supplier, the PCB designer, the board fabricator, and the assembly shop is mandatory.
The following factors have a major effect on the quality and reliability of PCB assembly: pad design, via-in-pad (VIP) guidelines, via finishing, stencil design, solder paste requirements, solder paste deposition and reflow profile. This application report provides a starting point for understanding the current set of guidelines. It is strongly recommended that you perform actual studies in conjunction with your assembly house and board supplier to optimize the process.
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This application report focuses on circuit board design guidelines specific to the OMAP35x processor, with 0.4mm pitch on the bottom and 0.5mm pitch pads on the top for memory attachment, using package-on-package technology. Experience has shown that PCB board design is the most crucial aspect of PoP design due to the extremely small pad pitches. Also, not all assembly houses can build such small pitch assemblies or properly mount the memory on top of the processor.
Figure 2 shows a 0.4mm pitch processor and its matching memory at 0.5mm pitch.
Guidelines for the assembly of PCBs that use the PoP package are covered in the companion article to this document, PCB Assembly Guidelines for 0.4mm Package-On-Package (PoP) Packages, Part II, which will be referred to as Part II throughout the remainder of this document. Included are assembly options and suggestions to use when qualifying and working with your assembly sites, either internal or contract.
This section discusses the BGA package and the circuit board immediately below the BGA. The guidelines do not cover all aspects of circuit board design, nor is it a discussion on how to use your favorite CAD tool.
Design of circuit boards for fine-pitch BGA packages at 0.4mm and smaller is more of an art than a science, due to the lack of published data. In many cases, 0.4mm pitch pcb fabrication technology is considered proprietary and/or advanced and may require that a non-disclosure-agreement (NDA) be executed. Therefore, the material in this document will age and go out of date quickly as more assembly houses adopt smaller pitches and PoP assembly.
Since this is a rapidly evolving technology, spend some time reading the huge number of articles, papers and company presentations on all aspects of fine-pitch board design.
When the 0.5mm pitch design guidelines were created, it was stated that the long standing rules for circuit board design were no longer applicable at these small geometries and a new batch of guidelines were established. With the release of 0.4mm pitch BGA packages, the new rules needed to be modified. This was determined through the design and assembly of TI developed evaluation module (EVM) boards and a new circuit design called the BeagleBoard.
The BeagleBoard is referenced extensively throughout this document and Part II. Be sure to obtain the Gerber files for this board at http://beagleboard.org. The files will be an invaluable reference during the design of your board. Also be sure to check for updates to this document that can be found on the beagleboard.org website as well as all TI websites for OMAP35x, Application Report Section.
It is strongly recommended that you plan on performing your own experimental layouts and prototype runs before committing to volume production to determine if: your suppliers can handle the device, you may have to change circuit board vendors, and/or find assembly shops with better equipment.
One important aspect of 0.4 mm design is the need for close cooperation among the various entities that are involved in the board design, fabrication, and assembly.
Successful design and assembly of complex, fine-pitch circuit boards is a Team Sport. The days of tossing circuit diagrams over the cubical wall to the board designer who then tosses them to the assembly shop are gone. Today’s board design requires a team approach and the entire process, from component selection to assembly requires careful coordination.
The typical team is composed of four different members representing the four major steps in product fabrication: the device supplier (chips, passives, mechanical), the PCB designer, the PCB fabrication shop, and the PCB assembly shop. There may be more members or some members may do more than one job.
Each of the team members brings their own experiences and design guidelines to bear on the task. As a result, it is not uncommon to find conflicting guidelines. These conflicts must be resolved prior to the start of work. Unresolved conflicts will result in poor assembly yields at best or 100% failure at worst. Constant and frequent communication is the key to resolving conflicts and everyone must be in the loop.
Get to know your team members and be sure to have frequent meetings as the project proceeds from design through production. It will be money and time well spent.
Board fabrication cost estimates vary widely. There are two key parameters that go into the overall quote cost:
Several board design and layout options are described with some options featuring a lower price. However, the lower price assumes that a relatively large number of boards are being fabricated. For smaller volumes, or for prototyping, the cost may be the same or even higher for some options.
Also, be wary of quick quotes that seem out of line. In many cases, the board vendor may be using outdated design rules causing a relatively high dollar quote for what might now be standard technology. Press them hard for ways to lower the cost. And be sure to obtain several quotes or get a second opinion.