Package crack

Figure 9 shows the principal stresses at the chip edge with the delamination advancement. We understand that tensile stress and maximum shear stress at the chip edge increases as the delamination advances. When complete delamination occurs, tensile stress and shear stress have reached their highest values. It can be concluded then, that the maximum possibility of package cracking occurs when there is complete delamination. 

This package cracking phenomena must also be related to aluminum pattern deformation and passivation layer cracking. 

Increasing the adhesion strength of a molding encapsulant and Si-chip interface is another method to prevent the package cracking at the chip edge. 

If package cracks are suspected, a dye penetrant test should be performed. In this test, a fluorescent dye is applied to the package surface and then wiped clean. Examination of the part under UV-light reveals residual dye in package cracks. This test is applicable both to hermetic parts and plastic-encapsulated microcircuits (PEMs). 

Visual Final seal Open circuits package cracks package seal problems... 

Popcoming A plastic package crack or delamination that is caused by the phase change and expansion of intemaUy condensed moisture in the package during reflow soldering, which results in stress the plastic package can not withstand. 

Today, almost 50% of the semiconductors encapsulated in plastic packages are made for surface-mount assembly that subjects the devices to a considerable thermal shock during the soldering process. Within a few seconds, the internal package temperature rises to 215-260°C and the moisture absorbed by the plastic encapsulant and the organic adhesive evaporates explosively. This sometimes results in package cracks that start at the interface between the chip and the die pad or in delamination within the die attachment layer. To investigate the relationship between the chemical structure of epoxies and this so-called popcorn elfect , a series of polyfunctional resins has been evaluated [5]. They include new experimental epoxy novolacs whose chemical formulae have been previously displayed [4]. 

Recently, so called package crack attracts attention. Package crack occurs as a result of the abrupt evaporation of moisture trapped at the interface of mold... 

Figure 7. Package cracking mechanism induced by vapor pressure.

Figure 8. Prevention of package crack by forming PI layer on the back side of die pad.

Table 4. Dependence of Package Crack on PI Chip Coat...

Material Adhesion Tg a) Water Absorption (WtX) Package Crack ... 

PI chip coat is effective for the prevention of passivation crack, A1 slide and package crack of semiconductor devices. Low thermal eiqpansion PI can be regarded as the most suitable material for this application. 

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