Technology with the Environment in Mind

Materials Technology for Environmentally Green Micro-electronic Packaging

HALOGEN-FREE PACKAGING MATERIALS INITIATIVE

As part of Intel's broad strategy to support an environmentally sustainable future, Intel is introducing environmentally conscious HF and Pb-free packaging at the 45nm CPU and 65nm chipset technology nodes. HF packaging materials introduced by Intel include several materials such as molding compounds, underfill materials, and substrates. The scope of this section of this paper is limited to HF-compliant substrate technology. Historically, components and printed circuit boards (PCBs) have used nHF flame retardants, which have been the subject of an environmental impact debate for a number of years. Primary concerns regarding nHF flame retardants include bioaccumulation and toxic dioxin formation during recycling. Intel's drive to meet HF requirements in substrates is to substantially reduce the Br and Cl levels in the substrates to meet HF requirements, which are currently being established by industry consensus.

Table 2: Comparative core material properties
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A key component in Intel's HDI package that requires conversion to HF is the substrate, which is the focus of HF enabling in this paper. Typically, nHF substrates contain Br and Cl-based compounds in the core material, the buildup dielectric layers, the solder resist, and the PTH plug material. To enable HF substrates, each of the above mentioned materials was changed over time. A schematic cross-section of a package is shown in Figure 21.

Figure 21: Assembled substrate schematic, labeled with key material items that required a change to HF
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Intel has several years of HVM experience with HF dielectric, plug and solder resist materials, and hence the final push to enable HF substrates required a change to the core material, wherein the HF core does not contain brominated flame retardants. HF core material candidates, which meet Intel's assembly and reliability criteria, have been identified, and these are presented in the next sections.

HF Core Material Selection

Challenges

The ideal HF core material is one that can serve as a "drop-in" solution, with material and reliability properties that meet or exceed those for nHF core material. By close matching of material thermo-mechanical and electrical properties, the degree of change to substrate manufacturing, assembly, board-level reliability, and performance can be minimized. Table 2 shows a sample comparison of nHF vs. HF core material properties, where x1, x2 refers to CTE in the x direction below Tg (x1) and above Tg (x2). As indicated in Table 2, the thermo-mechanical and electrical properties of selected nHF and HF cores were similar. This enabled a relatively smooth conversion from the standpoint of assembly, 2LI reliability, and electrical performance.

The mechanism for flame retardency in nHF vs. HF core materials is different, due to differences in the flame retardant used in the core. nHF cores typically use a brominated flame retardant, wherein the Br reacts with combustion reactant species, suppressing reaction propagation and creating a layer of char, both of which help to stop the fire. In contrast, HF core materials typically use a metal hydrate as the flame retardant, wherein the metal hydrate releases water to cool the polymer and simultaneously creates a char passivation layer:

Metal hydrate + heat → char + water

In practice, use of HF core materials in Pb-free packages can be challenging, because the HF core tends to undergo more decomposition/water release at Pb-free reflow temperatures (~260°C). This is due to differences in the flame retardant type and content in HF vs. nHF cores. This poses a challenge specifically for BGA component reliability, as a significant amount of moisture release from the core during repeated Pb-free reflows (for BA, board mount, etc.) can facilitate delamination in the substrate as shown in Figure 22.

Figure 22: Cross-section of HF substrate, which has suffered a delamination in the buildup layers
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To address this concern, a reflow accelerated test best known method (BKM) was implemented and used in the HF core material downselection process. The reflow accelerated test BKM incorporated JEDEC Pb-free Level 3 preconditioning (L3 precon) [8] with stepwise, additional Pb-free reflows (@ 260°C), to check for the delamination margin in the HF product substrates. Up to 15x additional reflows (beyond L3 precon) were run to test the delamination margin. Through careful material selection and screening, robust core materials, that met Intel's reliability requirements, were identified.

HF Core Material Results

HF core material selection required us to focus on substrate manufacturing, component and board-mount assembly, reliability testing, and performance. The results from these evaluations are presented in the following sections.

Substrate Manufacturing

From the substrate manufacturing perspective, the key challenges were selection of robust core materials, followed by mechanical drilling and flatness assessments of those materials. To select the most robust core materials, substrate suppliers were engaged and enabled with the reflow accelerated test BKM. Thorough evaluations were performed on various short and full loop test vehicles (TVs) with different designs, to understand the impact of core material and design on delamination reliability. Based on the number of reflows before the occurrence of delamination, the core materials with the most robust heat resistance were selected for further evaluation. These core materials were confirmed to be HF at Intel through ion chromatography testing, with the Br and Cl contents measuring <4 ppm. Core material drillability, as well as drill bit life parity between nHF and HF core, was established across the substrate supply base through drilling evaluations on the downselected materials. The parity in drill bit life ensures that there is no increase in drilling costs when an HF core is used. By measuring substrate flatness on incoming substrates, it was shown that HF core and nHF core units were equivalent across the substrate supply base.

Intel Assembly

The above trend carried forth through Intel assembly, wherein HF substrate flatness was the key assembly concern. Figure 23 shows comparative HF vs. nHF BGA ball coplanarity data for a 13x14mm TV. The data confirm that HF and nHF cores have similar flatness performance.

Figure 23: Comparison of HF to nHF package coplanarity for 13x14 mm TV
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Board mount of nHF vs. HF components was also studied, with results indicating similar board-mount yields for both nHF and HF units.

HF Substrates Component Reliability

Reliability testing at the component level involved use of Intel's new Pb-free L3 preconditioning plus up to 15x additional reflows at 260°C, to check for delamination margins. A number of HF core materials were dropped from consideration because of delamination margins. The selected HF core materials were robust during reliability testing, and for the given form factors/designs in Table 3, passed more than 10x additional reflows beyond L3 preconditioning before any delamination was observed. Reliability results were similar for substrates across Intel's supply base, indicating sufficient reliability transparency.

HF Substrates Enabled Reliability

Enabled reliability testing (component mounted on the board) also showed performance parity between nHF and HF core TVs as shown in Table 4. In shock testing (a.k.a. dynamic bend testing), neither nHF nor HF substrates showed cracks in any critical to function (CTF) BGA solder joints.

Flammability Rating

Due to concern about flame retardant decomposition during multiple Pb-free reflows for BGA products, and the potential implications of this for flammability rating, a check of the UL-94 flammability rating before and after extended Pb-free reflows was done. Table 5 shows the results, which indicate that both nHF and HF core materials were able to maintain a V-0 flammability rating after 10x Pb-free reflows.

Table 3: Example HF core component reliability performance for different form factors
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Table 4: Enabled BGA package reliability results for nHF vs. HF packages
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Table 5: Core material UL-94 flammability rating after extended Pb-free reflows
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Electrical Performance

Lastly, from a performance standpoint, nHF and HF components were tested side by side to determine the electrical impact of an HF core. Test results confirmed no impact on maximum operating frequency due to the use of an HF core, and electrical performance parity between nHF and HF was achieved.

In summary, a careful choice of HF core materials enabled Intel to introduce and ramp HF-compliant substrates that met assembly processing requirements as well as use condition component and board-level reliability requirements while maintaining the electrical performance of the package. Additionally, the selection of HF core materials with similar properties to nHF core materials enabled Intel to use existing recipes for component and board-mount assembly. This reduces the impact on the factory, and it potentially minimizes the impact on board-mount processes at customer sites due to the use of HF substrates.

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