SMT Magazine

SMT-Nov2017

Issue link: http://iconnect007.uberflip.com/i/895789

Contents of this Issue

Navigation

Page 61 of 105

62 SMT Magazine • November 2017 can eliminate the reflections within the unused portion of the via, which acts as a stub. The por- tion of the via indicated by the purple arrow is not in series with the signal path, but instead acts as a stub. Therefore, a portion of the sig- nal is reflected back, creating an interference, which will degrade the high-speed signal per- formance. Hence, the purpose of the back-drill is to remove this "unused" portion of the via in order to eliminate the reflections for a cleaner signal. With increased complexity of PCB designs for high-end networking products, the boards thicknesses are typically >120 mils and signal speeds are reaching 25 GHz and beyond. For these types of designs, backdrilling of the VIP- PO structures becomes imperative. It is also a common practice to mix VIP- PO and non-VIPPO pad structures within a sin- gle BGA footprint, as indicated in Figure 3. The green lines indicate a high-speed signal trace (e.g., for differential pairs) on the outer layer. It is preferable from a signal integrity perspective, to route these signal lines on the outer layers of the PCB to take advantage of microstrip rout- ing which has faster propagation speeds than stripline routing. Hence, these BGA pads do not require the use of VIPPO. These non-VIPPO pads are highlighted in red. Without any VIPPO structure, a zero stub length can be achieved, which is an extremely attractive option for the signal integrity engineer. Moreover, addition- al routing space is gained underneath the non- VIPPO pad. Unfortunately, these types of mixed footprint designs have a propensity for manu- facturing defects during SMT assembly of BGA packages and can potentially expose the PCBA to field reliability risks if these defects escape manufacturing tests. Failure Mode and History As a consequence of these advanced PCB technologies and complex board designs, a unique BGA solder joint failure mode has emerged during specific assembly conditions. This failure mode occurs when the bulk solder separates from the IMC during or just prior to reflow. This failure mode is of particular con- cern because the discontinuity is so small rela- tive to the size of the solder joint itself that it cannot be detected via X-ray inspection meth- odologies. Furthermore, in many cases it is only a partial separation of the BGA solder joint and hence, it may not even be detected via ICT or functional test techniques. Without a robust methodology to screen for these defects, this presents an extremely high risk for potential es- capes to the field. Typically, this failure mode has been found on BGA packages with a 1 mm pitch or less BGA array and having a PCB footprint that in- cludes a mixed VIPPO/non-VIPPO pad design. The solder separation occurs when the compo- nent is subjected to a secondary reflow, either during top-side SMT for bottom-side compo- nents or during rework of an adjacent, or mir- rored, BGA component. Since the open occurs between the bulk solder and the IMC, it does not have the typical brittle solder joint frac- ture signature, which has a flat fracture inter- face through the IMC as shown in Figure 4. In- stead, this failure mode exhibits more of a hot solder tear or separation type of failure mode, as the solder separates from the IMC leaving it in- tact. [1,2,3] Figures 5 and 6 illustrate examples of both partial and complete solder separations. For these failures, the solder separation only oc- curs on the solder joints that use a VIPPO BGA pad and is typically adjacent to a solder joint(s) with a non-VIPPO BGA pad. In some cases, this VIA-IN-PAD PLATED OVER DESIGN CONSIDERATIONS Figure 3: Mixed VIPPO/Non-VIPPO BGA Footprint.

Articles in this issue

Archives of this issue

view archives of SMT Magazine - SMT-Nov2017