This is the 15th year that microelectronic polymer suppliers have met in Wilmington DE to discuss advances in their industry segment.
The theme of this years event, for permanent dielectric suppliers, appeared to be the offering of positive tone, aqueous developable dielectrics with low curing temperature.
Twenty years ago the high performance PIs had excellent mechanical and thermal properties but required > 350 C curing temperatures to be fully cured. The introduction of BCB by Dow Chemical in the mid 1990’s offered a lower curing temperature ( 225 - 250 C) while not quite achieving the thermal or mechanical properties of PIs, was vastly superior to epoxies in these categories.
There are many reasons to desire lower curing temperatures such as: (1) newer generations of memory being more susceptible to elevated temperatures; (2) fan out WLP which need lower curing temperatures for their RDL layers to be compatible with molding compounds and to reduce warpage in these epoxy matrix wafers; (3) the trend for thinner and thinner die requires materials with low curing temp to keep warpage down in general for these thinned wafers, some as thin as 50 um; (4) need lower stress materials when coating the current fragile generation of low K chip ILD.
Below is a Closer look at some of the newer offerings from permanent dielectric vendors.
The LT series is a low-temperature curing, positive-tone photosensitive PI coating with a 170- 200 C curing temp and resultant 13 MPa thermal stress. By using preimidized PI and introducing hard and soft segments in the polymer chain, they were able to keep the curing temp and the resultant residual stress low while sacrificing some modulus.
Residual stress vs elastic modulus
A complete list of the properties for the LT grade are shown below .
Toray LT series PI
Asahi Kasei presented data on Photo PI buffer coat material for FC-BGA with low K ILD. Stress on low K (ELK) layer over a temperature range from 235 C to 25 C (cooling after reflow soldering) was calculated and stress concentrations were observed under the edge of the bump and under the edge of the PI opening.
Stress concentrations in RDL structures
Thicker PI was shown to reduce stress on low K (ELK) and a higher PI Young’s modulus reduced stress on ELK as well. However reducing residual stress is required for thicker PI or you will get higher warpage.
Like Toray, balancing hard and soft segments is used to control modulus while the use of low acidity amines is used to control the cure temp and thus the stress. As can be seen in the BL series, increasing curing temperature results in better thermal stability, lower CTE, higher modulus and lower elongation and higher residual stress.
HD Micro described development of a photosensitive PBO with improved chemical resistance. They indicate that their standard PBOs (8820), under some conditions, show delamination and cracking due to swelling due to the use of phenolic cross linkers. Newly developed 8850 reportedly eliminates these problems.
Properties of HD Micro 8850 PBO
The JSR WPR series products are both 5100 positive type (phenolic based [cresol] with 70 nm rubber reinforcement and DNQ photo cross lnker) and 1203 negative type ( p- hydroxy styrene matrix with chemically amplified photo cross linker).
In keeping with the theme of the meeting,
- Low residual stress (~20MPa) - Low cure temp. (180~200C)
Properties of JSR WPR Dielectrics
Dow reported on new developments with their aq developable low temp cure BCB P6505.
Curing of BCB P6505
Properties of BCB 6505 vs Std BCB 4000 series