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58.3.9 PWB Pad Design The size and shape of the pad used on the PWB has a significant impact on the reliability of BGA solder joints. In general, the reliability is optimal when the pad size/opening on the package side is the same size as that on the PWB side. Deviation from the same pad size could result in a reduction of up to 25 percent in solder joint fatigue life.14 The side with the smaller pad size tends to fail earlier because the stress concentration is higher near the smaller pad. Generally, non-solder mask defined (NSMD) pads are used on the PWB for packages with solder pitches in the range 1.27mm to 0.8mm. As opposed to solder mask defined (SMD) pads, NSMD pads do not have a localized stress concentration around the solder joint, which tends to degrade the thermomechanical fatigue life of solder joints. (Fig. 58.16)
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FIGURE 58.16 Solder-mask defined (SMD) and non-solder mask defined (NSMD) pad construction. For optimum reliability, the pad size on the PWB should be kept within 80 to 100 percent of the wetted pad area on the package side. This allows the stresses to be vectored equally between both ends of the solder joint, and if the PWB pad is slightly smaller, it allows a slightly higher stand off height, which also tends to enhance solder joint reliability.
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For packages with solder joint pitches less than 0.8mm, a mix of NSMD and SMD pads is sometimes inevitable. At small pitches, it is difficult to route the traces from the pads in the typical dog bone fashion. Thus, to allow routability, the common power and ground pads are bussed together to form one large pad. The connected pads are SMD, whereas the individual signal pads are kept NSMD. In general, it appears that while SMD pads tend to improve the resistance of solder joints to mechanical shock loading conditions, they also tend to degrade the thermomechanical fatigue life of the joints. Therefore, when the choice of pad construction is made, the impact on both thermomechanical fatigue and shock conditions should be characterized. (Details of typical shock testing conditions are outlined in Chap. 59). It is important to note that the pad size specified in the fabrication drawings is not always the pad size that actually comes on all the fabricated PWBs. Actual measurements of pads specified at 12 mils NSMD could range anywhere from 9 to 15 mils. This is particularly critical for small pitch packages, because the solder volume is smaller and such variations in pad sizes could significantly alter the shape and reliability of the solder joints.A tolerance analysis such as worst case (WC) or root sum squares (RSS) should be conducted and a pad size specified such that it would not significantly exceed the pad size specified on the package side. Refer to the IPC-7351 specification for details of such tolerance analysis.15
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Finally, it is clear that one common theme in the foregoing recommendations is that the PWB pad size should be pegged to the package side solder wetted area. Across the industry, the challenge with this recommendation is that the package pad size/opening is not always provided by package suppliers in relevant datasheets and mechanical drawings. To address this issue, JEDEC revised the JEDEC Publication 9510 in 2004 to include nomenclature showing the pad size and construction on the package mechanical drawings.Adoption of this JEDEC nomenclature by package suppliers would go a long way in ensuring that the PWB design used by end users does not deleteriously impact the reliability of the package. Effect of Lead-free Conversion. The impact of un-optimized pad size selection is detrimental to both tin-lead and lead-free assemblies. Fine pitch components (<0.8 mm pitch), regardless of solder metallurgy are much more prone to pad size-related failures.
Heatsink Design Issues As PWB designs get more complex and more densely populated, the power dissipation from single cards can be on the order of 1000 W. A lot of heat needs to be transferred from individual cards, and from the chassis as a whole. Consequently, heatsinks are becoming larger and heavier. Heatsinks as large as 10 sq. in., weighing more than 1 lb are not unheard of. If not designed correctly, large heatsinks could transfer a significant amount of compressive load to the packages they are meant to cool.This compressive load could result in cracks in the package/ silicon, or cause the solder joints to deform and bridge.16 On the other hand, smaller heatsinks that are anchored to the package substrate could impart additional bending moments on the substrate that could negatively impact the reliability of the package.17 For bolt-down heatsinks, a significant amount of the compressive load comes not from the heatsink weight but from the bolt down mechanism itself. Tolerance variations in package height, warpage, heatsink stand offs, and so on only make the applied load worse. The attachment mechanism should be such that it can accommodate these variations in tolerance without transferring excessive load to the package. Some of the attachment methods that could help accommodate tolerance variations and minimize the amount of loading imparted on to the package are shown in Fig. 58.17. The trade offs of using each method are summarized in Table 58.2. Pressure-indicating film can be used to verify the amount of load applied on the package. The film contains tiny microcapsules of colored dye. These microcapsules rupture depending on the amount of pressure applied on the film, producing a pressure footprint. The results are not extremely precise, but can be used to estimate the amount of load applied, as a first pass approximation. To verify the impact of heatsink loading on package solder joint reliability, temperature cycling can be performed on packages with heatsinks and compared with control samples. However, there are two caveats: 1. The heatsinks tend to cool the packages during temperature cycling, thus producing improved reliability data. The improvement may simply be because the solder joints are cycled over a lower temperature range. Thus it is important to either thermally shield the heatsink fins or calibrate the thermal cycling chamber such that the solder joints run at the same temperature range as control samples; in spite of the heatsinks. 2. Packages thermal cycled under compressive load could undergo two failure mechanisms: cyclic loading (fatigue induced opens) and static compressive loading (bridges due to solder collapse). Temperature cycling can accelerate the cyclic loading portion, but not the static loading portion. Thus, simply performing temperature cycling and comparing with control samples may not suffice. The static-loading component should also be accelerated, for instance, by dwelling at a high static temperature.
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