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Sn-Pb Cu3-Sn (c)
FIGURE 58.18 Solder joint intermetallic formation. (a) The constituent material stack up in a BGA substrate land pad. (b) After the ball is attached, a number of intermetallics are formed between the copper pad and the SnPb solder joint. Kirkendall voids are known to form at the phosphorus-rich Ni layer and Sn-Ni intermetallic interface. (c) After board level reflow, the interface region thickens and several more intermetallics are formed.19 (Reprinted with permission from Renesas Technology Corp./ [2003] IEEE ECTC.)
Ni-P
P-rich IMC
Crack
10 m 1 m
FIGURE 58.19 Brittle Solder Joint Failure.19 A clear separation between the P-rich phase and IMC on the package side is shown. (Reprinted with permission from Renesas Technology Corp./ [2003] IEEE ECTC.)
COMPONENT-TO-PWB RELIABILITY
PKG Side
Ni-P
IMC PCB Side X1 0000 Magnified Alloy 1 Large Void Alloy 2
Alloy 3 Alloy 4 Kirkendall Void
Alloy 5
Alloy 6
X20000 SEM image
FIGURE 58.20 Intermetallics Formed In Solder Joint with Brittle Fracture.19 (Reprinted with permission from Renesas Technology Corp./ [2003] IEEE ECTC.)
It is important to note that brittle solder fracture does not always occur exclusively. Frequently, it can be accompanied with pad lift, substrate cracking, or other intermetallic-related failure. 2. Black Pad Corrosion Phenomenon21 This is a discoloration phenomenon caused by excessive oxidation of the Ni. This leads to increased contact resistance, which causes intermittent opens during electrical testing. It is believed that the black pad phenomenon is caused by impurities in the plating bath during the ENIG plating process. The impurities create pores in the gold plating, thus exposing the Ni to moisture and other contaminants. Consequently, the Ni oxidizes and manifests in varying extremes as a discoloration in the pad. Tighter process controls during the plating process and regular
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purging of the plating bath can significantly mitigate this corrosion phenomenon. Failures stemming from black pad can be distinguished from brittle fracture by the following features: visible discoloration of the pads and a lot-to-lot dependency in failures due to impure plating baths. 58.3.11.1.1 Effect of Lead-Free Conversion. Given these known issues with ENIG, many package suppliers have moved away from it for lead-free applications. However, several package suppliers still use ENIG with standard tin-lead and lead-free solder. There isn t enough data to show the effect of ENIG on lead-free solder, but preliminary results indicate that the failure modes seen with tin-lead solder would also be seen with lead-free solder. In general, since lead-free solders are stiffer than tin-lead solders, they have higher strain/strain rate strength than tin-lead solders. More detail on tests to evaluate failure strength and failure modes is provided in the next chapter. 58.3.11.2 Solder-On-Pad (SOP). This is also known as pre-applied solder because it involves pre-applying a small amount of solder on the package substrate pad before the ballattach process. Care must be taken to ensure that the height of solder applied on the pad does not exceed the thickness of the solder mask (Fig. 58.21).
Pre-applied Solder
Solder Mask
Copper Pad Package Substrate PWB Pad
FIGURE 58.21
Solder-on-pad schematic.
Exceeding the height of the solder mask would result in alignment/registration issues during the assembly process. As with ENiG, there are concerns with the mechanical robustness of SOP solder joints. The Ni in ENIG serves as a diffusion barrier for the copper metallization. In the absence of Ni, copper diffusion occurs much faster and two intermetallics are known to form in SOP solder joints: Cu6Sn5 and Cu3Sn. Under high temperature aging conditions, these intermetallics are known to grow in thickness. Thus, if tin-lead or lead-free parts with SOP are aged at high temperature (either in lab testing or in field conditions), they could have a higher propensity for brittle fracture.22,23 By extension, one concern is that if solder joints can become brittle when aged over time, what would happen if the product has been operating in the field at high nominal operating temperatures, and it is subjected to shock conditions Would it fail early For instance, what if the product is a cell phone with an SOP based BGA used in it If the user drops the cell phone after sustained usage, is there a propensity for brittle fracture According to some publications on shock testing of leadfree assemblies (Cheng et al.),22 aged SOP joints subjected to shock conditions tend to fail relatively early. However, this has not been universally demonstrated. Moreover, the shock conditions to which hand held products are tested (~1500 G max), are much more aggressive than typical shock conditions for server environments (~500 G max). In tests performed on aged tin lead SOP samples subjected to lower shock conditions, the parts were not shown to fail due to brittle fracture, even though the presence of some Kirkendall voiding was observed (Mei et al.).24 However, the amount of Kirkendall voiding observed was not as pervasive as that reported by Cheng et al.22 (Fig. 58.22)
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