IFTLE 208 ECTC part 3: Thermal Compression Bonding – STATS, Toray, Qualcomm
By Dr. Phil Garrou, Contributing Editor
Before we continue our look at key papers from the 2014 IEEE ECTC Conference, the latest on the potential sale of STATSChipPAC (SCP). In June SCP, announced that it ended talks with one party (thought to be ASE) while discussions with other potential bidders were continuing.
Bloomberg now reports that the Chinese chip-testing companies Jiangsu Changjiang Electronics Technology and Tianshui Huatian Technology are considering bids for SCP and that a deal for SCP, valued at ~ $ 1B could be reached early next month. [link].
Semiconductors have been labeled a “strategic industry needed for China’s economic development and national security” by the Chinese Govt. China announced increased financial support for the industry and plans to set up a national investment fund according to Bloomberg.
Continuing our look at key papers from the 2014 IEEE ECTC conference
The advent of 2.5 and 3DIC has caused revisions in the way area array bump interconnect is carried out. The packaging hierarchy has traditionally been for BGA balls (~ 500um) to connect packages t boards and for C4 bumps (100-150um) to connect chips to packages.
Chip stacking with requirements for tighter pitch would prefer to have copper to copper connection, but such thermo-compression bonding currently requires 350-400C, 200kPa and most importantly 30+ minutes which makes high volume manufacturing impractical. Therefore the industry has adopted the micro-bump and the copper pillar bump for such mating.
Sematech has put out the following cartoon to show approximately when technologies have to be changed based on required pitch.
Most OSAT roadmaps show that standard solder reflow can be used down to ~ 40um after which compression bonding must be used to avoid shorting.
A closer look at these mucro bumps (see below) usually reveals a Ni barrier layer on a copper pillar capped with Sn/Ag solder.
Such fine pitch connections make it difficult to underfill and have driven the refinement of pre applied underfills [both film (called non conductive film, NCF) and paste (called non conductive paste, NCP) ]. These technologies only work when one has good manufacturing control over both solder reflow and underfill cure.
Let’s look at some issues concerning such interconnect that came up during ECTC 2014.
Y. Jeong detailed the “Optimization of Compression Bonding Processing Temperature for fine pitch copper column devices.” They examined thermal compression with non-conductive paste which they call TCNCP. The heat transfer from heating source to bumps must be tightly controlled in order to achieve the optimized bump temperature for the purposes of melting and soldering. To minimize voiding issues such as air entraps, a very short time window for curing has to be used in the NCP process. They determined the key parameters as shown below.
They concluded that “to have a successful bond, one of the most important keys is to obtain an optimized temperature profile which considers the ramp up/down speeds and times. The ramp up/down speeds and times affects the NCP flow behavior, void creation, and residual stress of the final product. In this regard, the peak and dwell time shall be precisely controlled to provide enough time for melting and soldering of bumps with substrate pads.”
Y. Liu of Qualcomm examined “Filler entrapment and solder extrusion in 3DIC Thermo-compression uBumps.” Qualcomm indicates that filler entrapment could negatively impact electromigration in the solder joint.
The main reason for filler entrapment is reported to be premature cure of the pre-applied underfill caused by not fully optimized process condition and/or bump geometry. Once curing initiates, solder wetting is no longer able to push out filler and underfill from the joint due to the increased viscosity of underfill.
They also find solder extrusion from the side of the ubumps and conclude through examination of large arrays of bumps that the extrusion appears to be random.
T. Nonaka of Toray described “High Throughput thermal compression NCF Bonding.” Torray points out that thermo compression bonding suffers from throughput issues because of the process flow shown below which requires the bonding head to cool between chip placements.
They propose a logical process change where the die are all placed and then cured and reflowed at once (gang bonded) as shown below.
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