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Package Reliability and Integrity Improvements for a Thermally Enhance Non-Conductive Die Attach Adhesive for ASIC Devices on Exposed Pad Packages
Keywords: Die Attach Adhesive, Non-conductive, Exposed Pad
Polymers being used in a plastic- encapsulated integrated circuit (IC) package exposed to a humid environment absorbed moisture and expand resulting to a so called delamination failure. Weak or imperfect adhesions between the interfaces of the mold compound and adhesive unto the leadframe surface are often the main sources of these failures. In response to automotive requirements and to ensure excellent package reliability and integrity, delamination in all interfaces should then be eliminated. Thus the primary objective of this work is to fulfill the no delamination criteria in all interfaces after moisture soak. To satisfy these requirements, activities includes leadframe design improvements, surface enhancement and bill-of-material (BOM) changes. On the existing exposed pad SSOP 36-lead package using a non- conductive adhesive, the main defect observed were the delamination seen at the mold compound and die attach adhesive to leadframe paddle interfaces. In order to initially address the issue, leadframe surface roughening treatments as well as design enhancements were introduced to enhance mold locking features. Subjected to a 30C and 60% RH (relative humidity) for 192 hours (MSL 3 soaking condition) and to a 500 temperature cycles (TC) -65C/+150C conditions were made to verify the effectiveness the improvements while considering the effect of the adhesive bond line thicknesses as well. Thru Scanning Acoustic Topography (SAT) results, Table 1 then shows that the improvements are effective regardless of bond line thicknesses. To speed up the experimentation, the next round of evaluation will cover the existing non-conductive die attach material and three (3) new candidates with different material properties as shown in Table 2. Based on materials thermal conductivity and compatibility, together with the improved leadframe material, the delamination performance after 85C and 85% RH for 168 hours (MSL 1) and 85C and 60% RH for 168 hours (MSL 2) soaking conditions and 1000 temperature cycles were checked. SAT results in Table 3 validates that with the existing material the improvements were proven not effective to higher moisture soak conditions which is in contrary with the new adhesives considered. Representative SAT photo of the units that passed and failed the delamination criteria is shown in Figure 1. Material compatibility then plays an important role in achieving improved package reliability. To further choose the best material for this specific package type, thermal impedance (the sum of its thermal resistance and all contact resistances), Zth values of the new materials were taken as shown in Table 4. With Sample C, having the lowest thermal impedance value (with higher thermal conductivity value than the existing material) was chosen to be the best material in the activity. With the above results, SSOP36-lead Exposed Pad packages using a non- conductive adhesive with high thermal conductivity can now achieve MSL 1 without any form of delamination in all interfaces and even up to 1000 temperature cycles. It is then recommended to perform another confirmation or large scale validation runs prior qualification builds to verify the effect of material lot to lot delivery, variability and process consistency. Furthermore, another set of evaluations will be conducted for conductive adhesives on exposed pad packages.
Alvin Denoyo, New Product Development Manager
ON Semiconductors
Carmona, Cavite
Philippines


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