Thursday, November 7, 2013

IFTLE 168 EMPC Part 2: FC Market; BLR of BoP WLCSPs; Chip Embedding; Temp Stability of Molding Compounds

Yole – FC Market & Tech Trends

Rozalia Beica, newly appointed CTO of Yole Developpement, examined the FC marketplace. The FC market is currently growing at CAGR of 19% as a result of expanded use in memory, consumer electronics and mobile phones. In 2012 bumping capacity of 14MM 300mm equiv. was in place accounting for 81% of all “mid end” capacity.

FC technology is being reshaped by the demand for Cu pillar bumping (CPB) and microbumps which are both quickly becoming mainstream. CPB is expected to show 35% CAGR over the period 2010 – 2018.

FC capacity is expected to grow in the next 5 years in response to demand from (1) 28nm CMOS application processor (APE) and baseband (BB) applications;(2) next gen DDR memory and (3) 2.5/3DIC.

                                                FC Bumping and CPB Forecast 2010 – 2018
 ASE – Board Level Reliability of BoP WLCSPs
There hsas been reduction of  the production cost for WLCSP packages for the past years. Today, many OSATs are working on further cost reduction with customized WLCSP package designs that are optimized for specific market needs.
For example, omitting the UBM layer on smaller WLCSP devices can reduce costs and may still meet the market requirement on package quality and reliability. Omitting the UBM requires 25% less process steps, from 4-mask process to 3-mask process.
ASE reports on the BLR performance of a 3-mask bump on polymer (BoP) WLCSP design vs  a 4-mask BoP WLCSP design for 0.4mm and 0.5mm ball pitch using  tin/silver/copper ( SAC) and SACNi (Ni doping) solders and reports on failure analysis.
     WLCSP (a) 4 mask process with UBM; (b) 3 mask process without UBM;  
(c) failure modes for 3 mask process
The polymer material can be polyimide (PI) or Polybenzobisoxazole (PBO) with thickness of  5um to 7.5um. In most BOP WLCSP packages, ASE states that PBO is the preferred material for better stress compliance, and hence better board level reliability.
For 3-mask WLCSP design, there is no UBM. The solder ball is directly attached to the redistribution layer, using polymer 2 to define the pad opening. Therefore, the electrolytic plating copper thickness for RDL needs to be sufficiently thick to avoid any problems due to Cu consumption during SnxCuy intermetallic (IMC) formation during thermal ageing. For these reasons the Cu RDL thickness is increased from 4um, on 4-mask WLCSP, to about 8um on the 3 mask process to ensure a reliable solder joint. the thickness of polymer-2 also needs to increase to 12um polymer-2 thickness to ensure line coverage. A 12um thick polymer-2 layer creates processing challenge for PI or PBO, and the thermal stress or residual stress after high temperature curing needs to be carefully controlled to guarantee the integrity of package structure.
After board level reliability test, failure analysis was performed to confirm the failure mode. The failure modes were classified as failed at PCB side Mode A, failed at component side Mode B and solder fracture Mode C. In the failure analysis, we found that BLR failure modes are governed by shear rate applied to the tested samples. High shear rate test, like drop test, tended to fail at the component side with IMC fracture (Mode B2) or residual solder on pad (Mode B3). But, for slow shear rate test, like temperature cycling test, the fail tended to occur at the solder joint (Mode C). They concluded that 3-mask the WLCSP does not change the failure mode in either temperature cycling test or drop test.
They conclude:
- BLR temperature cycling performance is governed by the WLCSP device size (DNP). The bigger the DNP, the worse temperature cycling lifetime. This was evident for both solder materials used in this study, even though the larger device has a larger solder joint size, and there was a larger difference between SAC405 devices than SACNi devices.
-  In general, the 3-mask WLCP has worse BLR performance than 4-mask WLCSP.
-  SACNi solder gives improved BLR Drop test performance (characteristic life, and first fails) for both 4-mask and 3-mask WLCSP devices.
-  They found the same failure mechanism and failure modes on 3-mask WLCSP as 4-mask WLCSP.
 Chip Embedding at IMS
 Ultra-thin chips  (less than 50 μm thick) can be assembled by either on flexible films; i.e. chip-on-foil technology or by embedding them inside the foil. Initial work by the Institute for Microelectronics Stuttgart (IMS CHIPS) dealt with attempts to glue attach Chipfilm dies onto flexible foil substrates. They have now described research with < 20μm thickness die with a two polymer (BCB and PI) ultra-thin chip imbedding approach.
Polymers used for embedding should be flexible and at the same time strong enough to keep the chip firmly embedded. To achieve an optimal solution for the desired process IMS used a combination of polymers where BCB serves as the embedding polymer for ultra-thin chips and the PI as the  reinforcement polymer. The X sectional structure  is shown below.

Two Polymer Embedding of Ultra-thin Chips
The PI reinforcement layer provides strong yet bendable reinforcement for the entire chip stack. The BCB embedding polymer provides excellent electrical properties, low moisture absorption, compatibility with the interconnect metals and fine pitch patterning compatibility.  The process flow is shown below. An initial “adhesion lowering layer” is initially coated on the wafer to allow for package removal once the process is complete.
Ultra Thin Chip Embedding Process Flow
The figure below compares the warpage of the free standing 4.6 x 4.6mm 20um thick chip to the chip once bonded to the lower BCB layer of the structure.
NXP- High Temp Capabilities of Epoxy Molding Compounds
Packaging for automotive and other high temp applications and for GaN and SiC high temp technologies requires  packaging be able to withstand high temperatures. Epoxy Molding compounds (EMC) will therefore be subjected to steadily higher ambient temperatures, possibly reaching their intrinsic limitations. Currently,  few materials have been qualified to withstand ambient temperatures of greater than 175C.
NXP studied the degradation mechanism of three types of epoxy molding compounds (A): OCN-based, Tg greater than 150°C, (B): Biphenyl-based, Tg approx. 120°C and (C): Multiaromatic, Tg approx130°C.
When an EMC is exposed to air (oxygen) at high temperature, two competing chemical reactions take place. During oxidation, oxygen gets incorporated into the polymer chains and the resultant groups (e.g. carbonyl groups) can react with hydrogen atoms from neighboring chains, leading to an increase in cross-linking density of the EMC. Oxidation also leads to chain scission, where smaller more volatile molecules are liberated into the air. Both reactions lead to chemical shrinkage in the polymer. Thermal ageing in air leads to an increase in intrinsic stress (due to shrinkage) and a decrease in strength for the oxidized layer. When the intrinsic stress exceeds the strength of the EMC cracks form on the surface.
It is expected that some key thermomechanical properties will change after ageing, the main changes will probably take place only in the oxidized layer.
A number of (thermo)- mechanical properties were measured like the glass transition temperature (Tg), Storage Modulus (E’), Loss modulus (E”), CTE below and above Tg (CTE1 and CTE2) and the flexural strength. All of these properties degrade with oxidation .
Even before the onset of external cracks in the compound, it has been shown that ageing leads to a relatively porous and “open” oxidized layer.
Class C (Multi-aromatic) have the most stable performance demonstrating  that a high Tg does not necessarily imply higher thermal stability.
CMST (U Gent/IMEC) – Parylene as a Diffusion Barrier
CMST and co-workers have studied the use of Parylene [ poly(p-xylyene)] as a barrier material for biocompatible implants. The packaging of implantable devices needs to be miniaturized as well as the devices. The barrier layers should be hermetic , i.e provide a bidirectional diffusion barrier . Parylene C and N are prone to oxidation at temperatures above 70 C. Testing showed   limited water vapor barrier protection, adhesion issues with certain surfaces and limited copper corrosion (although only after 170 hrs). Copper diffusion is observed through a Parylene barrier layer . They conclude that Parylene  in combination with other layers might be a better solution. [IFTLE offers that NO Polymer is a hermetic barrier and as such should not be used as the main source of hermeticity for implantable devices. ]
For all the latest on 3DIC and advanced packaging stay linked to IFTLE……………….

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