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Micro Testing Thin Die

The Challenge

The need for micro testing thin die 100 micron and below is being driven by the increased demand for thinner mobile products with PIP (Package in Package) and SIP (System in Package) devices for smart phones and tablets. This combined with a need for ultra-thin die 50 micron and below for RFID tags ID cards and related products is presenting an emerging challenge for micro testing at the die level and bond testing interconnect at the packaging level. This article takes a look at how micro materials testing and advanced bond testing has evolved to provide solutions.

Micro Testing Thin Die

Inorganic semiconductors are brittle and their strength is greatly affected by the presence of surface flaws created by dicing and handling. These flaws manifest themselves as chips and scratches.

Die can experience high levels of stress due to CTE mismatch or flexure of the substrate or board that they are mounted to.

Flexural testing is ideal for assessing the impact of defects (cracks) and surface damage on the strength of brittle materials.

The standard way of assessing the unfavorable effects of dicing and grinding is to perform a bend test on a statistically significant population typically greater than 25 samples and this is normally done in accordance with industry standards such as the following:

 
 Standard Description
SEMI G86-0303 Three point bend test of die 
SEMI G96-1014  Chip (Die) strength by cantilever bending 
                                   

Different bend methods such as 3 point , 4 point , ring on ring and ball on ring (spherical bend) result in different stress distributions .The failure modes for 3 and 4 point bend testing are sensitive to both surface and edge cracks.

The 3 point bend method for the measurement of die strength is described in SEMI G86-0303.

Three point bend method

 

Micro bend – with a range of sample holders.

Micro bend   Micro bend

 

Testing conducted on the Nordson DAGE 4000Plus platform utilizing the Paragon™ for materials general user interface.

4000Plus Bondtester

 

Micro Testing Ultra Thin Die

Three point bend testing is not ideal for ultra-thin die 50 microns and below as their increased flexibility dictates that the supports should be close together causing practical difficulties with the test therefore testing in accordance with SEMI G86-0303 becomes inconvenient.

Micro testing ultra thin die

at h <0.1mm : L ≤ 2mm and L ≤ 50 h

Therefore an alternative method should be adopted which is the SEMI-G96-1014 Cantilever bend method. This particular method involves landing a shear tool (bending tool) on the surface of a work holder and then stepping back (up in Z) to a precise controlled test height before moving the Y stage towards  the transducer in order to apply a load to the face of the die.

Bending tool  Bending tool

3 stage positions

 

Young’s Modulus is reduced with die 50 microns thickness and below and therefore the traditional 3 point bend method becomes difficult to use.

Wafer thickness

 

Bond Testing Stack Die

Rapid advances in semiconductor packaging technology continue to drive bond testing capability. The testing of interconnects on stack die which are commonly used in SIP (System in Package) presents a number of new and exciting challenges.

One of these challenges is landing on compliant thin stacked die surface and accurately stepping back to a preset shear height before performing a shear test on a ball bond.

Overhanging die in particular can be difficult to bond test because of the complications such as thickness of the die (flexing during load tool landing and step back) and the small spacing between the bonds. An industry wide solution for the shear testing and pull testing of bonds underneath the overhanging die has yet to be perfected however with the aid of specialized  software the problem of die surface deflection associated with the load tool landing has been solved.

The Nordson DAGE Series 4000Plus has a software option that enables the operator to set a soft land step distance (measured in millimeters).

When landing the shear tool on a compliant surface the small downward force applied by the shear tool is enough to move the surface. After the tool has clamped and lifted (step back) to the designated shear height, the surface of the die will deflect upwards towards its original position. The resulting shear height relative to the surface is now significantly less than the step back height selected.

Having landed and clamped the shear tool, the XY table makes small scrubbing movements back and forth. The cartridge load cell detects the movements of the table whilst in contact with the surface and is raised in steps until no contact is detected. At this point the die has returned to its original position and the load cartridge will reset to zero and step back to the desired height before performing the ball shear test.

Bending image Figure 1 2 3 text 


Measuring the Deflective Strength of Thin Stacked Die

Stack die technology is also incorporated in memory products such as NAND flash memory, USB memory, SD and SRAM and as such it’s not uncommon to encounter overhanging die in many of these package styles. As a result there is an emerging need to be able to use bond testing to duplicate the downward force that a wire bonding capillary would apply during the bonding cycle on the overhanging portion of the die. This is typically a destructive test with the peak force being known as the “deflective strength”.

This particular application would involve a 4000Plus mainframe with a 500 Gram push/pull load cartridge with a bonding capillary fitted to the load cartridge.

Please see the below drawing:

Stacked thin chip bend test

 

Typical test results:

 Die thickness  Typical destructive force
 30 um  48 to 70 grams
 40 um  90 to 130 grams
                       

Summary

With the advent of new materials, technology and advanced packaging trends it’s critical that traditional bond testing techniques are evolved to accommodate more of a micro materials testing environment. Many new and diverse testing techniques are needed to meet the challenge and as a result there is no longer a divide between bond testing and materials testing, it’s all essentially micro testing and Nordson DAGE are ready to work with customers to find solutions.

Acknowledgements and special thanks to Dr Ian Mayes Nordson DAGE UK, Tadashi Torimititsu Nordson ATES Japan, Courtesy Tohoku University Japan.