The Challenges of Qualifying ‘Green’ Packages
NEW MATERIALS AND FAILURE ANALYSIS
The lead-free and halogen-free “green” initiatives currently spearheaded by the EU and Japan is forcing advanced packaging manufacturers to evaluate new materials that must meet stringent reliability and quality standards. These new materials affect failure analysis tests when qualifying green packages.
The primary objectives in creating green substrates are the elimination of brominated flame retardants from the laminates and the reduction of chlorine ppm from the soldermask. Popular replacements for flame retardants are metal hydrate, metal oxides, red phosphorous and nitrogen-based, organic FR. This can potentially cause hygroscopic swelling or deterioration in electrical performance.
Soldermask materials do not have a substantial amount of bromine. Of the ~2,500 total ppm of halogens, only 200 ppm is bromine, and 2,300 ppm is chlorine. Therefore, the focus during green conversion is reducing the ppm level of chlorine, which results in a soldermask that is more reliable and environmentally friendly. It is able to achieve this because chlorinated flame retardants release hydrochloric acid, which attacks adjacent copper circuits. By reducing the ppm of chlorine, the material is more robust at high temperatures. T-SCAN after moisture sensitivity level (MSL) 260°C shows delamination on the standard soldermask. A physical cross section confirmed that separation is between the soldermask and the laminate interface (Figure 1).
Figure 1. STD soldermask cross section with delamination.
An important issue during the conversion to halogen-free is keeping the green color of the soldermask. End-users prefer the green color, because it eliminates the need to readjust vision and equipment sensors. The green color of the soldermask is from the halogenated dye used (phthalocyanine green). Creating an environmentally friendly green-colored soldermask is challenging because the halogen-free dye is blue (phthalocyanine blue), however, a soldermask manufacturer formulated a green-colored, halogen-free dye by combining phthalocyanine blue with anthraquinoid.
Figure 2. TDR analysis.
A time domain reflectometer (TDR) analyzes electrically failed parts (open, short, leakage) and identifies point failure in the IC package - substrates, die, wire, and solder balls. This is a faster method than doing evaluation iterations and electrical test verification after decapsulation. Figure 2 shows an example of TDR analysis in which point of failure is determined to be within the substrate’s area.
Leadframes do not contain halogens. Due to lead-free requirements, however, design changes are necessary to make it robust at high temperature. Common failures are electrical opens caused by lifted wedge on the die paddle. This is a result of delamination of the mold compound to the die pad’s silver plating; silver is known to have poor adhesion with mold compounds. In a test, significant improvement of delamination after MSL 260°C is realized by reducing the silver coverage from spot-plated to ring-plated (Figure 3).
Figure 3. Ag spot and ring.
Another solution to enhance robustness is the use of leadframe with a special pre-plated finish (PPF) structure - Ni/Pd/Au, with silver strike on top. Test results for a SOIC package show quantum improvement on delamination performance compared to Ag ring-plated leadframes (Figure 4).
Figure 4. Ag ring (top) and special PPF (bottom).
Other factors that affect die paddle delamination include package structure, die-to-die pad ratio, contamination, surface topology, resin bleed, CTE, and material properties. Modeling software can be used to understand effects of material properties on temperature-related stress, reducing the number of evaluations.
Die attach materials rarely contain halogens. For green conversion, the material has to be assessed in terms of resistance to high reflow temperature and moisture.
There are several considerations when choosing the best die attach: thermal stability, low stress on the die, minimal out-gassing, minimal bleed-out, compatible with mold and leadframe/substrate interfaces, modulus, glass transition temperature (Tg) and low moisture absorption. Some of these properties may be significant on one package, and they may not be on another. For example, adhesion strength is important for small die sizes, whereas, for larger die size it could be modulus. Actual experimentation is the most accurate way to determine the best die attach material, however, the use of modeling software can help reduce the number of evaluations.
Figure 5. CSCAN (left) and TSCAN (right).
Common failure mechanisms are stress-induced delaminations. These include mold compound to die attach fillet, die back side to die attach, die attach to leadframe and separation within die attach material. T-SCAN must be performed to check for delamination beneath the die area. C-SCAN will not detect delaminations under the die if the IC package still has mold compound. Figure 5 shows a T-SCAN of a unit that has the die paddle fully delaminated. The corresponding C-SCAN photo shows no delamination on the die area. From these two photos, presence of top die delamination can be ruled out. Actual delaminated interface can be verified by physical cross section.
Standard molding compounds use brominated flame retardants. Environmentally friendly molding flame retardants used today include metal hydroxide and multi-aromatic resin (MAR). While bromine converts to a gas that prevents oxygen from getting to the flame, metal hydroxide releases water and absorbs heat resulting from the endothermic nature of the dehydration process. MAR is a self-extinguishing resin that forms a foamed layer to block oxygen. Due to its low cost, metal hydroxide-based molding compounds are mainly used on discrete packages such as SOICs and dual in-line packages (DIP). The downsides to using a metal hydroxide-based molding include reduced spiral flow and adhesion strength. MAR, on the other hand, is primarily used for applications that require flexibility in filler loading because of its high resistance to cracking. A problem with MAR is that it is relatively expensive. Phosphorous-based compounds also have been considered as alternatives. However, high moisture absorption make this an unpopular choice with most packaging companies.
All environmentally friendly mold compounds must survive JEDEC delamination criteria at lead-free reflow temperatures. To meet this requirement, manufacturers have increased the adhesion strength of the mold compound to improve its reliability performance. This has been a successful approach to attaining good reliability and performance. The down side is that it also sticks well to the mold chase, increasing assembly equipment downtime caused by frequent cleaning.
“Green” mold compounds also must be optimized for decreased wire sway during the mold process. Wire sway occurs when the mold compound pushes the wire during its flow in the cavity. This sway can cause the wires to short with the adjacent wires. “Green” mold compounds are more susceptible to a higher degree of wire swaying compared to STD mold compounds, due to the thick nature of the flame retardant replacements, as well as the added fillers. Wire swaying can be overcome by mold process parameter optimization and making adjustments to the mold properties (spiral flow and filler distribution).
Industry Standard Solder Balls
The industry standard 63% tin/37% lead solder balls are now being replaced with a tin, silver, and copper alloy. The most widely used alloy combination is 3 to 4% silver, 0.5 to 1% copper and tin, with a melting point of 210° to 221°C. The main drivers here are cost, melting temperature, and board-level performance of solder joints during temperature cycling.
An alloy that has a melting temperature above 230°C is generally not preferred, because of the stress induced on the components and substrates during the assembly solder ball reflow process.
Removal of lead from tin/lead is the focus of lead finish ‘green’ conversion. The lead-free solution uses 100% tin plating or pre-plated leadframes (PPF). PPF leadframes come in different composition and stack up, ranging from 2 to 4 layers of nickel, palladium, and gold. A typical 3-layer PPF has nickel on the first layer, palladium on the second layer, and gold on the top. The advantage of using PPF leadframes is that it shortens the assembly process because no post plate is required. When using the PPF leadframe, the forming machines must be checked to ensure that it is able to accommodate the thickness of the PPF leadframes. Most PPF leadframes typically are thicker than post-plated versions. If this is not taken into consideration, the plating can crack during the forming process (Figure 6). Moreover, solderability and wirebondability for PPF must be checked, because the multiple (Ni/Pd/Au-Ag) plating process could introduce variations in plating thickness.
Figure 6. Sn 100% (top), and special uPPF.
The 100% tin solution still battles industry’s concern on whisker formation. It is believed that the uneven intermetallic layer between the copper and tin (Cu6Sn5) creates stress that results in whiskers. These whiskers can cause electrical shorts in electronic assemblies. The lack of known methods to proactively identify whisker-prone products makes the 100% tin solution risky for high-reliability applications such as satellites. Countermeasures being used include post-plate reflow, tin over nickel, optimizing plating bath chemistry, and annealing.
With the push from the semiconductor manufacturers for complete ‘green’ IC packaging growing and becoming a global initiative, assembly and test providers must find the right combination of package materials. Simply offering lead-free versions of packages is not enough. For each package material component, tests must be performed on the various ‘green’ and lead-free materials to determine their compatibility, workability, and reliability. To minimize evaluation iterations, a material properties review must be done first to determine whether the material components satisfy ‘green’ requirements.
Reliability performance varies with package type and reliability test conditions. Full reliability assessment is necessary to ensure that the material set meets the test requirement. During this transition stage, it is important to establish foolproof controls to distinguish the ‘green’ from the standard materials to prevent the use of wrong materials.
ALLAN TORIAGA, director of operations, Assembly Engineering, and JEAN RAMOS, manager, Package Development, may be contacted at Advanced Interconnect Technologies, 1284 Forgewood Ave., Sunnyvale, CA 94089; e-mail: firstname.lastname@example.org, email@example.com.