Jan/Feb, 2024
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Fine-Pitch Direct Die Attach Without Thermal Compression
By Andrew Stemmerman and John Yundt, SunRay Scientific, Inc. F
lip-chip die and die-to-die bonding, from dense to fine pitch typically require sol-
der balls and underfills. Under - fill and/or edge encapsulant is often utilized to provide addi- tional mechanical strength and stress reduction. The result is a complex
assembly process flow. Localized placement of Anisotropic Con - duc tive Adhesive (ACA) or Aniso tropic Conductive Film (ACF) for specific components typically involves the fine-pitch use of thermocompression bond- ing, an additional process step that could also be damaging to thin silicon. Another drawback for tradi-
tional interconnect materials is relatively slow processes, limiting the utility of such technologies. SunRay Scientific has devel-
oped a novel approach, using a pressure-less and low-tempera- ture magnetically aligned aniso - tropic conductive epoxy (ACE). First, ferro-magnetic particles dispersed within an epoxy are coated onto a substrate. The ferro-magnetic particles form Z- axis magnetically aligned col - umns, fixed in place during the die-to-substrate cure process without any pressure applied.
Single Adhesive Application This technology simplifies
the assembly process to a single adhesive application, which pro- vides both electrical interconnec- tion and mechanical reinforce- ment. No additional underfill material is needed. Fine pattern- ing is not required as the entire area of the component target location is deposited with epoxy. The device alignment process is more forgiving relative to solder ball-to-solder pad alignment. Z- axis columns align after compo- nent placement, magnetic pallet exposure and cure is achieved. Thermal or UV curing meth-
ods complete the component attachment without any thermo- compression (cure method is epoxy formulation dependent). Thermal curing occurs within the 176 to 320°F (80 to 160°C) temperature range. SunRay then attached a 126-
pin Land Grid Array (LGA) bare die to a polymer semi-rigid multi- layer substrate. The use of the substrate resulted in multiple challenges for bare die attach. The non-planarity of the conductive circuit pads was one issue. The electrical resistance variation between pads had to be minimized for optimum performance. This takes the LGA and sub-
strate subassembly to the next level — a large area 8 x 10 in.
See at IME West, Booth 3549
Target overlay of quartz substrates (left), ACE deposit before bond (center), and bond before z-axis alignment and cure (right).
(203.2 x 254 mm) circuit board populated with four of the sub- assemblies plus components of various sizes and functions. This larger assembly involved attach- ing all the multiple components and subassemblies in one ACE attachment process. Passives ranged in size from as small as 0201 up to 2220. Other devices
were a 26-pin SMT connector Continued on page 59
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