Figure 1: Technical Session 3 just before start
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How costly is a failure?
Last week I attended the Utah Expo & Tech Forum sponsored by the Intermountain Chapter of SMTA. It was a very well put together event with excellent speakers and relevant exhibitors. One Technical Session in particular was called Water, Water, Everywhere Understanding the tremendous damage that moisture causes to electronic components, how to control it, and what FA equipment do we use to analyze it, by Steve Greathouse from Plexus Corporation. In this session Steve talked about the failures associated with moisture accumulation in both the parts and the boards in automated assembly. It was a real eye opener.
The two most frequent root causes of board failure are water and mismatched CTE. If you haven’t given this much thought, you will understand why all plastic encased silicon parts come hermetically sealed with a humidity sensor and desiccant package in each envelop. The molding compound used to encapsulate the silicon die immediately begins to absorb moisture when it is removed from the package.
Popcorn is not just served at the Movies
If a board build is stopped midway through, the reels must be removed and stored in a humidity controlled chamber until the build resumes. Not doing so risks catastrophic failure in the reflow oven or latent failures that will only manifest after temperature cycling in the field. The reason for this damage is apparent when you realize how similar an IC chip is to a kernel of popcorn. When they reflow process takes place, the board and parts are exposed to elevated temperatures that turn any moisture into steam. That steam can heat buildings, move huge locomotives and destroy IC’s. The comparison is apt because when moisture destroys a chip by blowing the top off it is called popcorning.
Where is Stretch Armstrong when we need him?
Another failure that can take place is the stretching of wire bonds. When water expands and lifts the lid of the chip it pulls at the wire bond. A wire failure occurs when the wire necks down as the lid pulls away from the chip. If the wire does not fail the silicon below the bond can fail or the bond can pull away from the silicon.
The second major cause of failure is CTE mismatch. When you cycle the temperature on a board full of parts you are going to put tremendous stress and strain on those parts. Many people incorrectly believe that parts are at their maximum stress at elevated temperatures. The exact opposite is true. Parts are at the maximum stress at low temperatures. At high temperature, near the reflow temperature, parts are at the minimum stress point.
To solve all these problems the temptation is to drop the humidity as low as possible. That will reduce the moisture absorbed into the parts, but it will dramatically increase failures associated with ESD. Keeping a humidity around 50% will reduce the ESD problems and proper management of parts kept in controlled atmosphere when not being placed is a good compromise.
Moisture can damage more than your parts
Steve told a story of a very expensive multilayer board that was ripped apart when placed in the reflow oven. The 26-layer board was built in stages. The core was made first and then set aside while the top and bottom layers were worked on. By the time the final board was produced, the core had absorbed substantial moisture that lifted the top layers. In some cases, this can be seen as blistering. In other cases, it is reminiscent of the movie Alien.
How many times do you need to temp cycle something?
Some companies boast that they have tested their products through 1000 temperature cycles without a single failure. Unfortunately, they have not learned anything. To produce useable data, you must have failures. If it takes 2000 or 3000 cycles to get those failures then that is what you need to do.
Failures damage more than parts and devices. They can damage our names and reputations. It is important to understand best practices and the why’s behind what we do.
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