Printed Circuit Design & Fab - May 2008 - (Page 42)

QUALITY CONTROL FIGURE 5. An uncollapsed BGA ball (a) results from an improper, too cold thermal profile. A wrong or excessive thermal profile (b) causes BGA bridging. FIGURE 4. Properly created assembly drawing eliminates confusion, answers questions, and reduces board defects. core thicknesses to prevent innerlayer registration problems. Similarly, applying correct board surface finishes is of the utmost importance, especially for Pb-free product. Where OEM cost is an issue, Pb-free HASL or ImAg may be the right way to go for a Pb-free board. However, ImAg has a tendency for corrosion. In this case, pending corrosion would not be immediately apparent. However, over time, it can begin to occur in the field, creating one or more defects and subsequent product failure. Therefore, a better choice could be ImAu. Again, experience and knowhow play into situations like this to make the right call. Fabrication notes also cover IPC class, according to the board that is going to be built, such as Class 1, 2 or 3. Mistakes can occur if the fabrication class is not properly called out in the fabrication notes. It’s also important to document the percentage of warpage allowed. Defects can arise from excessive warpage (bow and twist), improper etchback, plated versus non-plated hole errors, and incorrect mechanical tolerances of drill bits. Plus, drilled holes must have sufficient tolerances so they become full holes without creating half moons or having stubs. These and many other fabrication factors set the stage for defects. It’s also crucial to define the number of panel stackups for the drill and routing stages. For example, in a sixboard stack, specified drill and routing tolerances may be achieved on boards 42 1, 2, 3 and 4, but not 5 and 6, due to mechanical drill limitations. To avoid this, the number of panel stackups is defined as a process. Defining processes like this within well-documented fabrication notes provides an extra measure of assurance for eliminating defects. AOI can also be defined as a process for inspecting innerlayers, especially for high-count layer boards using extra thin prepregs. Assembly. Multiple factors can eliminate board defects at assembly stages. Like fabrication notes, there should be a comprehensive assembly drawing detailing all the necessary assembly processes. For example, a process can clearly spell out that all components, including BGAs, are to be machine placed, as well as any rework or ECO callouts and the use of any special process. FIGURE 4 shows an example of a properly created assembly drawing, which can eliminate confusion, answer questions, and reduce board defects. Assembly processes must be repeatable. The first article is important at the assembly level as well, so technicians can check for polarities, missing components and other key areas. Planning must be conducted to determine and document the processes that need to be defined, the machines needed, engineering change orders (ECOs) and use of special equipment such as arbor press for press-fit connectors, AOI machines or flying probe testers. Documentation is important for technicians in the field who need to read and decipher ECOs that deviate from original build. Likewise, rework instructions, if any, need to be clearly spelled out with solid quantitative data for measurement and verification purposes. At times, instructions can be issued in an assembly drawing to avoid board defects, which could be process-related issues. Also, depending on how progressive an EMS provider is, post reflow inspection can be specified as part of the process for all BGAs, CSPs and QFNs, as opposed to at QC at the end of assembly. At this point, actions can be taken to avoid board defects such as improper board reflow, poor orientation, wrong thermal profile and improper flux activation, among other problems. This is important to eliminate defects. If the thermal profile is incorrect and the board does not heat properly, the result will be cold solder balls on the BGAs, CSPs or QFNs (FIGURE 5). If the thermal profile is too hot, bridging can result. During first article post-reflow inspection, while boards are going to reflow, the process can be stopped; it can be changed and a set of one or two boards as second articles can be run to correct the process. But if some are not caught during the process, they are found at the end of it when it can be too late. A shipment could be missed, or excessive rework required, taking more time, resources and extra dollars. Thus, it is important for EMS providers to build QC steps within the assembly/manufacturing process. It’s also important for OEMs to specify process-related issues as part of the assembly drawing to avoid defects. PCD&F ZULKI KHAN is president and founder, NexLogic Technologies, Inc. (nexlogic. com); zk@nexlogic.com. MAY 2008 PRINTED CIRCUIT DESIGN & FAB http://www.nexlogic.com http://www.nexlogic.com

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - May 2008

Printed Circuit Design & Fab - May 2008 Contents Our Line Market Watch Around the World Happenings ROI EMC For the Real World PCB East Conference Brochure Positive Plating Don't Let your Signals Stub Their Toes Improve PCB Layout With Skill Utility Programs The Next Generation Design Tool Challenge Thermally Conductive Microwave Materials PCB Dielectric Degradation in Lead-Free Assembly Applications A Tale of Two Trade Shows Eliminating Board Defects Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - May 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.