Package Cracking

 

Plastic Package Cracking

  

Plastic package cracking is the occurrence of fracture(s) (see Fig. 1) anywhere in or on a plastic package.  Over the years a vast range of mechanisms that lead to package cracking had already been characterized.  Mechanisms vary from one package type to another, and some may even be unique to certain package groups only.

 

Most of the known mechanically-induced package cracks come from assembly.  Worn-out DTFS blades and punches can result in large deflections of the leads and tie bar during processing, creating excessive stresses at their plastic interfaces.  Cracks occur if these stresses exceed the molding compound's fracture strength. Tight deflashing coupled with excessive package mismatch can also create excessive stresses at the bottom package during deflashing, leading to cracks.  Single-stage lead forming also creates excessive stresses at the lead-to-plastic interface. 

     

Leadframes with no anchor holes and with poorly designed bonding fingers are prone to lead pulling, which is usually preceded by cracking at the lead-to-plastic interface.  Debris underneath the package during DTFS can produce large bending stresses on the package,  which lead to cracks if they exceed the plastic's fracture strength.  Inadequate package nesting during DTFS can result in similar package cracks.

           

Overcuring of the package renders it brittle, and may make it more vulnerable to assembly-induced cracking. Insufficient bond line thickness due to inadequate die attach material dispensing may result in cohesion failure within the die attach material, which can lead to package delamination and cracking.

                

Figure 1. Photo of an SOIC with a corner crack

 

Plastic surface mount devices (SMDs) may crack because of the intense pressure build-up generated by the vaporization of the internal moisture inside the package during solder reflow.  Moisture is often absorbed by the package from the environment during storage prior to the solder reflow.  This mechanism is known as 'popcorn cracking,'  since a popping sound may actually be heard during the moment of fracture.

    

Delaminations between the die and the plastic and between the die paddle and the plastic precede package cracking.  Prior to cracking, the vaporized moisture will expand the gap between the delaminated plastic and the die paddle to form a dome or bulge on the package. The excessive stresses are relieved only after the fracture occurs, which usually originates at the corner of the die paddle.  The crack propagates to the surface when the maximum bending stress exceeds the molding compound's characteristic fracture strength. 

     

Factors that affect popcorn cracking tendency include the solder reflow temperature, the moisture content of the package, the dimensions of the die paddle, the thickness of the molding compound under the paddle, and the adhesion strength of the molding compound to the die and leadframe.  A package becomes more robust as its die paddle length decreases and its bottom plastic thickness increases.  Recent data however indicate that thin packages with large die paddles do not absorb moisture, and are therefore less vulnerable to popcorn cracking.

   

Plastic Package Delamination

   

Plastic delamination refers to the disbonding between a surface of the plastic package and that of another material.  Plastic delamination may therefore occur at an interface of the plastic and the leadframe, die, die paddle, or die attach material.  It also means the loss of adhesion between the plastic material and one or more of the other materials.  In a plastic package, the sources of this adhesion are the chemical bonding between the molding compound and the other materials' surfaces, and the differential contraction of the materials.

 

Contaminants on the surface of the leadframe, die, or die paddle can prevent good adhesion with the plastic material and lead to delamination. The use of incorrect leadframe texture, dimensions, and design can also reduce adhesion strength. The use of molding compounds with excessive mold release agent can also lead to delamination. Excessive mismatches between the thermal coefficient of the plastic and those of the leadframe, die, and die attach material can also result in delamination. Insufficient bond line thickness due to inadequate die attach material dispensing may result in cohesion failure within the die attach material, which can lead to package delamination and even cracking.

 

Packages of plastic surface mount devices (SMDs) may delaminate internally, if not crack, because of the intense pressure build-up generated by the vaporization of the internal moisture inside the package during solder reflow (see Plastic Package Cracking for more details). These type of delamination occurs between the die and/or die paddle and the plastic, and always precedes package cracking.

                 

Seal Cracking

   

Solder seal cracking is the occurrence of fracture(s) (see Fig. 3) anywhere in the solder seal of a ceramic package that uses a combo lid, e.g., sidebrazed, LCC, and JLCC packages.  Most solder seal cracks may be attributed to defects in the seal, which in turn are due to a poor solder sealing process. Mechanical defects such as voids, incomplete coverage, and poor filleting, and chemical defects such as seal oxidation or corrosion, weaken the seal and make it susceptible to cracking.

 

A sudden and large change in package temperature can lead to solder seal cracking too, especially if there are defects in the seal.  Such sudden changes in temperature cause tremendous thermomechanical stresses at the ceramic-to-metal interfaces of the package, due to large differences between the coefficient of thermal expansion of ceramic and those of the metals used in the package. For instance, inadequate pre-heating can result in solder seal cracks during solder dipping.

 

Poor package design or condition can also cause solder cracks. The use of insufficient seal path widths or inadequate seal preform can result in narrow seals, which are more likely to crack than a robust seal.

 

A lot of solder seal cracks are just secondary effects of package cracking.  Interestingly, some seal cracks even look worse than the primary failure mechanism, namely package cracking, which can be invisible without staining the package. A thorough check of the package for microcracks must therefore be conducted everytime a solder seal crack is analyzed, no matter how gross the seal damage looks.

 

Seal glass cracking is the occurrence of fracture(s) anywhere in the seal glass of a Cerdip package.  Most seal glass cracks may be attributed to impact stresses.  A common cause of impact stresses is end-to-end banging between units from improper handling of tubes.  Advanced stages of seal glass cracking from impact stresses can lead to base-cap separation (see separate article on 'Base-Cap Separation').

 

Seal glass cracking may also be aggravated by seal glass defects such as base-cap mismatches and offsets, glass overcure, glass undercure, and excessive glass etching from the cleaning processes prior to leadfinish.

 

Ceramic Package Cracking  

  

Ceramic package cracking is the occurrence of fracture(s) (see Fig. 2) anywhere in or on a ceramic package.  Ceramic cracks can be caused either by thermomechanical or by purely mechanical means.  The distinction between cracks due to thermomechanical causes and those due to purely mechanical causes is not always easy though.

            

A sudden and large change in package temperature causes tremendous thermomechanical stresses at the ceramic-to-metal interfaces of the package, due to large differences between the coefficient of thermal expansion of ceramic and those of the metals used in the package. For instance, inadequate pre-heating can result in ceramic cracks during solder dipping. Packages with castellations are vulnerable to this mechanism, since the expansion of the metal inside the castellation during solder dipping tends to split the ceramic around it, initiating a crack that can easily propagate across the package.

                

Impact loading is the most common source of package cracks in ceramic DIPs. Impact loading is the sudden application of force on a body.  Ceramic units inside metal tubes can easily crack if the tube is accidentally dropped to the floor, especially if the floor is made of concrete.  End-to-end banging from tube-to-tube or equipment rail-to-rail transfers can also result in cracks.  High-pressure air drying that propels one unit into another has even been found to cause base-cap separation.  Data also suggest that improper tube packing can lead to microcracks that can propagate through subsequent handling. Massive units dropping to any hard surface of an equipment can also suffer microcracks.

              

Figure 2. Photo of a cerdip with a corner

package and seal glass crack

          

Poor package design or condition can also cause package cracks.  Sidebrazed units with rhino horns are highly vulnerable to package cracking and chipping during end-to-end collisions.  Some sidebrazed packages need their plating bars removed by grinding.  The grinding process can produce both a recession and a protrusion at the package end.  Such a protrusion is called a rhino horn, and acts as a stress concentrator when a unit collides with another during tube-to-tube or rail-to-rail transfers.  This stress concentration can result in cracks that originate that the corner of the package where the rhino horn is present. Package defects like microcracks, voids, geometrical aberrations, and the like all act as stress concentrators.

 

Poor equipment set-up or debris under the package during back-end mechanical operations like lead trimming may possibly cause cracks, although this has become rare over the past years.

   

Base-cap separation is the detachment of the cap from the base of a cerdip package due to high-impact shearing between the cap and base.  Base-cap separation is basically a failure of the glass seal of the package, even though the impact loading occurs at the ceramic package itself.  Base-cap separation may therefore be defined also as a total fracture of the seal glass all around the seal path, resulting in the separation of the cap from its base.

 

The most common cause of base-cap separation is end-to-end banging. Units with excessive longitudinal mismatch or offset between the base and cap are therefore more susceptible to exhibit this failure. The use of extremely high pressure for blow-drying units can propel units into one another, and lead to base-cap separation.  Improper tube-to-tube or rail-to-rail transfers can also lead to base-cap separation. Units inside metal tubes that are dropped accidentally to a concrete floor may also suffer base-cap separation.

   

Links:  Package DelaminationSeal Cracking Ceramic Package Cracking

Package FailuresPackage Crack FA Flow

 

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