The LTS solidification phase happens in a range of temperatures that, coupled with the change of warpage magnitude and shape, can elevate the risk of hot tearing. In most cases, the peak reflow warpage and solder solidification warpage reduce in magnitude, depending on package type and design construction. The dynamic warpage of electronic packages can be different when assembled onto the PCB using LTS versus eutectic solder such as SAC systems. This higher warpage reduction seen in PBGA is attributed to the higher sensitivity of the package to temperature as a result of the higher volume of mold encapsulations used. The majority of PBGA packages shows a warpage reduction from 40% to 250%. The even lesser change for large packages is mainly due to the constraint from the lids coupled with thicker constructions. As for FCBGA with lids, the percentage of warpage reduction is about 20% to 50%. This reduction seems reasonable as the results of 30% to 50% warpage reduction were reported. For FCBGA and FCBGA MCP packages, the majority of the packages considered show about 20% to 100% warpage reduction and a few show warpage increases as well. The majority of the package warpage change is as low as 10% to about 100%. This, however, reflects the change of warpage percentage and not the absolute warpage magnitude. This extreme change is due to the fact that the warpage using LTS is a few orders of magnitude higher than the warpage at 260☌. The markers denote the trending of the warpage change where not all types of packages experience a reduction of warpage as a result of adopting LTS.įor PoP and FBGA packages, the warpage change ranges from negligible impact to a factor of ~1000%. Figure 4 shows the percentage of warpage change for the various families of package technology. Hence the percentage of warpage change in adopting to an LTS solder system is relevant to the SAC peak warpage reflow at 260☌. Hot tearing defects are a critical concern and solutions may exist to enable LTS in SMT processes as an attempt to mitigate yield and reliability associated with high dynamic warpage behavior.Ĭomparing the magnitude change can be a challenge as each package has its unique dynamic warpage characteristic. It can also occur without Bi stratification if the solidification temperature overlays with the package substrate package warpage shape inversion temperature. Hot tearing is thought to be caused by an interaction of Bi stratification at the package interface coupled with dynamic warpage of the FCBGA package as it cools from peak reflow temperatures. One of the key defects highlighted in adopting BiSn-based LTS paste is the hot tearing defect of mixed SAC-BiSn alloy solders which was observed in FCBGA joints located at the die shadow. The project has been primarily focused on the mixed SAC-BiSn solder joints (assembly of components with SAC solder balls attached to packages with LTS paste screen printed on the board for SMT), but also covers some evaluation of homogenous solder joints (LTS solder ball with LTS solder paste). INEMI's BiSn-Based Low-Temperature Soldering Process and Reliability (LTSPR) project, started in 2015, is studying the possible impact of various LTS pastes on SMT processability, mechanical shock and thermal cycling reliability. The resulting lower reflow temperatures required inherently reduces energy demands and may favor the dynamic interaction between package warpage and solder phase formation. There are several significant variables that have an impact and they all distill down to synchronization of temperature, warpage and phase of solder.Īdvances in alloyed solder materials have led to the introduction of low temperature solders that melt and solidify at a range of temperature from 130☌ to 200☌. For example, packaging materials, size and construct have an impact as does the PCB layout and construction. Some electronic packages - such as plastic ball grid arrays (PBGAs) - demonstrate a higher sensitivity to temperature changes and the resulting warpage can result in defects such as solder bridging and non-contact opens, which can impact yield.ĭynamic warpage of a package is affected by several factors – not just the soldering temperature profile. The higher processing temperatures can aggravate dynamic package warpage to more extreme levels. SAC process conditions require higher reflow temperatures and result in lower ductility than traditional lead-based solder systems. Over the last decade, lead-free Sn-Ag-Cu (SAC) solder has been widely adopted in components and board assembly. Electronic package warpage must be managed to ensure the final package can be easily assembled on a board.
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