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Moisture sensitivity levels for plastic-encapsulated ICs

Posted: 28 Feb 2014  Print Version  Bookmark and Share

Keywords:plastic encapsulated microcircuits  printed circuit board  popcorning  reflow  JEDEC 

When plastic-encapsulated microcircuits are on the assembly floor, waiting to be mounted on a printed circuit board, they will absorb moisture from the environment. Water molecules percolate into the mold compound. They typically come to rest at material interfaces, such as the interface between a mold compound and the die, or the interface between a mold compound and a metal lead frame. Water molecules may also be trapped in the die attach material between the die and the die paddle.

During reflow, any water inside the component package is likely to reach the boiling point, at which it flashes into steam and increases its volume more than 1600 times. The sudden increase in volume can cause cracks and other anomalies within the component package. This phenomenon has often been called "popcorning" due to the sound of the ICs "popping" on the board like kernels of corn.

The majority of these anomalies do not cause immediate electrical failure. But during the service life of the component these gap-type anomalies may collect both moisture and contaminants that percolate into the mold compound. The cracks may also increase in size through simple thermal cycling. Some of these anomalies will eventually cause an unexpected electrical failure when they break a connection.

Figure 1: Floor life times, conditions, and soak requirements for the eight moisture sensitivity levels.

The joint IPC/JEDEC standard 020 was written to prevent the intake of moisture and reflow damage during the assembly process from leading to future electrical failures or other reliability issues. The current version, IPC/JEDEC J-STD 020D.1, classifies plastic encapsulated microcircuits into eight categories, depending on the time they can spend in an assembly atmosphere before they intake enough moisture to cause anomalies to occur during reflow (figure 1). Components in Level 1 are very resistant to moisture intake and can tolerate unlimited exposure to the assembly environment at a temperature up to 30°C and at a relative humidity up to 85%. The other seven levels have the same 30°C temperature limit, but have exposure times ranging from one year down to the number of hours stated on the label. Handling components according to their moisture sensitivity levels (MSLs)reduces dramatically the number of field failures caused by internal structural anomalies.

The MSL for each type of component is determined experimentally because the design, materials and construction all influence the component's rating. To determine the MSL for a given component type, engineers follow this general sequence:

1. A batch of components having a specific specific is imaged with an acoustic microscope to determine whether the components contain any existing cracks, delaminations or other packaging anomalies. Sonoscan has worked closely for years with IPC and JEDEC on the details of acoustic imaging for determining MSLs. Internal anomalies seen at this time formed during manufacture of the component.

2. The components are baked to remove any existing moisture.

3. The components are "soaked" according to the schedule in figure 1. There is also an alternate accelerated soak schedule for levels 2a through 5a.

4. The components are then placed loosely, without touching each other, in a clean, dry, shallow container. The printed components are then passed through simulated reflow three times. Three passes are used because two-side printed circuit boards will be reflowed twice and may later undergo the thermal extremes of rework to replace failed components.

5. The components are removed from the container and tested electrically. This test merely determines whether a component is still electrically active.

6. The components are then imaged acoustically a second time in order to observe, non-destructively, any anomalies that may have appeared during reflow, as well as the growth of previously existing anomalies.

7. If the acoustic images indicate that the components as a group failed the criteria in 020, then they did not pass that MSL. The acoustic images can also be used as a guide for physical sectioning of the components for further validation.

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