Printed Circuit Design & Fab - April 2008 - (Page 39) INNERLAYER FABRICATION away, revealing the copper substrate. Breakpoint should take place sufficiently early in the developer chamber that complete pattern formation is guaranteed, with no residual material remaining, but not so early that resist feature line width is reduced by dissolution in the developer chemistry. While the exact breakpoint settings used in different processes vary, typical values range from 30% to 60%. Etching and Resist Stripping Proper control of etch rate and etching factor is achieved through a combination of chemical concentration management and equipment maintenance. For most etching, temperature and spray pressure far outweigh any other factors in determining etch rate. Although resist stripping does not directly affect the pattern formation, it may sometimes have an indirect impact on process yield. Incomplete stripping or copper surface oxidation after stripping can lead to a non-uniform surface appearance. When finished innerlayers are examined by AOI, these surface irregularities can trigger false defects. If the equipment settings are adjusted to eliminate the false signals, AOI may fail to detect actual defects, leading to scrapped panels at final, a very expensive and undesirable problem. BGA Bulletin, continued from p. 48 outside the center of the ball pad array has a relatively little benefit to routing due to the lack of space. Any benefits depend on the design rules. You could move the vias offcenter somewhat in order to form columns and rows, but for every additional route channel opened on one side, it will close a route channel on the other – the net result being the same number of route channels across the entire BGA. However, if the design rules are such that differential pairs cannot be routed together between the through-vias, and a little more space is required, then shifting through-vias slightly might make sense. In FIGURES 3 and FIGURE 4, note that by shifting the vias to the left on one column and to the right on the next column, you gain 0.17 mm in one and lose 0.17 mm in the other. These values could vary, depending on your ball pad size and clearance rules. This option might be useful if you have some critical signals that require greater spacing within the BGA breakout area. Another reason for shifting through-vias while using Quad Dog-Bone patterns is to maximize the amount of plane fill on the mount layer. Again, any benefit is dependent on the design rules. Via-in-Pad Adhesion Promotion As global assembly has shifted towards lead-free processes, and with more complex product often requiring multiple laminate operations, the increased stresses placed on the innerlayer copper/laminate interface have in many cases exceeded the capability of existing laminate materials. Even when new materials designed for lead-free assembly processes are used, unexpected failure modes can require the reevaluation of material qualification and test methods. As an example, delamination failures of boards have been reported in assembly after storage in ambient conditions for 3 to 6 months. Prior to lead-free, storage for this period of time would not have been considered a potential problem. For these applications, some laminate systems based on dicy cure have been found to not perform as well as novolac systems. In addition, surprisingly complex interactions between adhesion promotion systems and laminate material formulation have been reported for oxide, reduced oxide and oxide replacement. This has led to new approaches to adhesion promotion process qualification, including evaluation of adhesion performance on a wider range of copper substrates, and use of more severe screening tests. Test combinations must certainly include copper foil and plated copper (as would be seen on plated innerlayer products) combined with the full range of laminate material types that will be processed. Performance tests should include assembly simulation testing immediately after lamination, after pressure cooker exposure and also after extended room temperature storage. PCD&F This pattern has a simple solution, which is to add a throughvia in the center of each of the desired BGA ball pads as seen in FIGURE 5. Advantages over quadrant dog-bone. ■ If you have a ground or power plane on the BGA mount side, the fanout via pads allow a continuous plane fill under the BGA. ■ If you do not use the mount layer for a plane, then you have an additional routing layer for the BGA – albeit a surface layer, which may cause impedance problems when using high-speed nets. Disadvantages over quadrant dog-bone. ■ There are no additional route channels in the center column and row. ■ There is less room for capacitors and resistors on the opposite side under the BGA, since the fanout via array is full. If the BGA has unused pins and you do not add fanout vias for them, there will be available room for components. ■ There will be a slightly higher cost for filling the vias and ensuring a smooth surface when soldering the ball pads. ■ There is some risk of BGA soldering problems (delamination or pop corning) with via-in-pad when using lead-free solder. An experienced assembly company should be able to manage this risk. Better Methods Although through-via stackups generally provide the lowest cost fabrication, the large size of the via severely limits your ability to maximize route density and therefore leads to excessive layer count. The next article will clearly demonstrate the benefits of using blind and buried vias. PCD&F CHARLES PFEIL is an engineering director for Mentor Graphics, Systems Design Division; charles_pfeil@mentor.com; mentor.com/pcb. PRINTED CIRCUIT DESIGN & FAB 39 REFERENCES 1. Stouffer Industries, Inc. Step Tablet at stouffer.net. SIMON LEE is Regional Marketing Manager Imaging and Final Finishes – Rohm and Haas Electronic Materials Asia Ltd.; tslee@rohmhaas.com. BETTY T.Y. XIE is Product Manager Imaging – Rohm and APRIL 2008 http://stouffer.net http://mentor.com/pcb
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