When overexposed Moisture-sensitive components go through the solder reflow cycle, the entrapped moisture will turn into steam which can cause internal cracks and delaminations. These internal defects will not necessarily cause an immediate component failure but they can affect their reliability.

Rebaking these assemblies will not fix the problem. To evaluate the risk you could perform some reliability tests on the affected units. The simplest solution is to remove and replace the suspect BGA components with dry ones.

For more information on this topic I invite you to consult the MSD Knowledge base on our web site.

If the components surpassed allowable moisture exposure limits prior to reflow, which sounds likely, then they may have incurred internal damage during reflow.   Typical moisture-induced defects include micro-cracking or delamination inside the component body, due to excessive pressure exerted by absorbed water molecules during reflow.   There is usually no external evidence that can be detected through visual inspection, X-ray or other standard inspection techniques.

Such defects are often latent; in other words not severe enough to cause immediate failure.  This would explain why the units passed initial testing.  However, out in the field and over time a small crack can propagate into a larger one, suddenly and unexpectedly (think crack in your car’s windshield), resulting in intermittent performance issues, early life failure, or diminished long term reliability.

Re-baking the assemblies now will not “undo” any damage that occurred during the initial reflow process. Your safest bet is to replace these components with ones that were stored and monitored properly prior to reflow.  If they are expensive then an alternative would be to send them to a lab for acoustic microscopy.  An acoustic microscope is the only non-destructive way to check inside the component body for cracks and delaminations.  Such an analysis would confirm for sure whether the components need to be replaced.  

Instead of using heat (a known enemy of electronics) in an attempt to drive off moisture, it may be more advantageous to use a vacuum chamber in order to actively evaporate any residual water. Heating can put undesirable thermal stresses on electronic devices, which can lead to premature failure.

Try using a batch-type plasma cleaning system or other type of vacuum chamber, and pump the chamber down into the several-hundred mTorr range for 15 to 20 minutes (without engaging any plasma or heating, of course.) We find this is usually sufficient to remove residual moisture from electronic components.

Anecdotal of course, but we’ve had success this way recovering cell phones that have been accidentally dropped into water. The key is to immediately remove the battery from the phone in order to ensure there are no short circuits before the water can be thoroughly removed via this vacuum process.

Whether or not you’ll see short- or long-term failures in your components due to moisture exposure depends on many factors: Did any shorts occur while the moisture was actually present? Did the moisture deposit any contaminants (such as salts or acids) that can subsequently cause shorts, corrosion, or delamination? Did the moisture induce any other modes of failure into the component?

A bit of a disclaimer: For mission-critical electronics, such as those used in medical devices, military electronics, or avionics, the safest action is always to replace components which are known to have been exposed to moisture.

We recommend that the component manufacturer’s specs be followed. If the moisture-exposed final assemblies passed inspection, that is not a guarantee that premature failure will not occur in the future. Manncorp’s T40W series of baking dry boxes offer a superior method of storing MSDs (moisture-sensitive-devices) at ≤ 5% RH.

Not only do they offer the dehumidification benefits, they can also be used to reset the floor life of parts exposed to ambient moisture-laden environments. With low-temperature baking at 40°C, parts are safely stored for an unlimited amount of time without concern regarding moisture or oxidation issues. For more information, access https://www.manncorp.com/dry-boxes/t40w/

Any damage that might have been done by processing expired MSDs cannot be reversed by baking the assemblies.  And the fact that the assemblies passed in circuit and functional testing is not proof that there was no damage during reflow. 

One of the most critical facts to understand about the dangers of improperly managing Moisture Sensitive Devices is that the micro-cracking and related defects are difficult to detect.  They are frequently on the underside of the components and typically manifest themselves in the form of field failures two to six months after the fact.

There is no substitute for a comprehensive MSD management process just as there is no substitute for a comprehensive ESD management process.  And lead free processing raises the risk of not having procedures in place even higher.