Powerful Prototypes: Moisture Sensitivity—What’s the Risk, and What Can You Do About It?

I recently traveled to New Orleans, Louisiana, for a week of beignets, fried food, and urban exploration. It’s a good thing that parts of the exploration came in the form of walking as some level of exercise was needed to compensate for the lack of greens and heavy emphasis on the word “fried” that went along with most of the food. I’m convinced that New Orleans is pretty close to a vegetable-free zone (unless you can call a beignet a vegetable).

Outside of the dietary concerns, the humidity was my only complaint. I’m a wimpy West Coaster, as far as weather is concerned. Even with temperatures in the 50s and 60s (Fahrenheit), I could still get all sweaty in the 85% humidity. That surprised me. And something else related to humidity (see what I did there) that surprised me when I first learned of it was that plastic can absorb moisture. I had always thought of plastic as being a moisture barrier. Certain types of plastic are, but not necessarily the plastics and resins typically used to construct electronics gear.

Some components are more sensitive to moisture than are others. IPC—Association Connecting Electronics Industries and our industry standards association—rates levels of sensitivity based on how long the component can be exposed to an unprotected environment before it’s at risk of having problems when being soldered.

Ordinary levels of moisture aren’t a problem during operation of a finished product. Although, most devices have humidity maximums or at least operational sensitivity isn’t our primary concern as an assembly house. Our area of concern is during the high temperatures of a solder reflow oven or wave solder machine.

Electronics components absorb moisture very slowly over time, but when thrown into a reflow oven of a few hundred degrees Celsius, that moisture may flash boil and try to escape at a much faster rate than the component package can facilitate. If you’ve ever made popcorn and have compared a popped kernel to a non-popped one, you will see an extreme case of what can happen. I’ve seen chips with holes where big chunks of the package exploded off. It’s less dramatic, but even worse when the damage is subtle and not easily seen by the eye.

The package can split just enough to let the moisture out and let contaminated air back in. The split can also break or crack wire bonds and lead frame structures, or the PCBs may delaminate just enough to break or crack via columns. Such damage can pass inspection and test at the factory, but lead to early failure in the field.

So, what can you do about it? How do you protect your parts and products against the “popcorn” effect? Note: “popcorning” is an industry-accepted term for this phenomenon.

First, it’s important to understand what you’re up against. IPC has a standard (imagine that) that covers moisture sensitivity. J-STD-020 ranks moisture sensitivity in eight levels from 1 to level 6, and it’s not that I can’t count; Levels 2a and 5a are stuck in the middle (Table 1). These levels are based on the amount of safe processing time you have between removing the component from its moisture-safe packaging and soldering (Figure 1).

Benson_Table.jpg

Table 1: J-STD-020 Levels 1–6.

Benson_Figure_1.jpg

Figure 1: Digi-Key labeling and packaging, showing where the moisture sensitivity rating is.

Level 1 means that the component is essentially not moisture sensitive. A Level-6 component is so sensitive that it will show the number of safe hours or minutes on the package. Most parts seem to be Level 3, which can be open and exposed for a cumulative time of 168 hours. The actual safe time may vary based on your local humidity. For all of the details in between, you can download the current version of the J-STD-020E standard at jedec.org.

When you buy your moisture-sensitive components, they should come in a moisture-barrier antistatic bag with an indicator card and a little baggy of moisture-absorbing desiccant. The best approach with these components is to leave them in the original, unopened bag. Your manufacturer will use what is needed for the job and properly seal up the rest.

If you want to open and reseal, you'll need the humidity indicator card that will look similar to Figure 2, including the parts, desiccant, and a thermal-sealing gizmo. Open the package; take out the parts you need; put the remainders, desiccant, and card back in the package; and reseal it. Sealing it with tape won't do the trick. Then, you would count the time that the components were in the open air toward the cumulative open time.

Benson_Figure_2.jpgFigure 2: The moisture-sensitivity indicator card will be inside the package with the parts.

PCBs from a good supplier will come sealed with desiccant also. The risks and standards are not as clearly defined as for components, but proper handling is just as important. No matter how good the design, if the PCB isn’t in good shape, you won’t be able to build a reliable product.

In lieu of a standards document, I checked with the PCB experts at Sunstone Circuits. With the PCB, you need to be aware of two issues: the surface solderability preservation, as I wrote about in my March column, “Proper PCB Storage: Top Three Hazards,” and moisture absorption, which may create a risk of delamination.

According to Matt Stevenson, VP of sales and marketing at Sunstone Circuits, “For electroless nickel immersion gold (ENIG) and hot air solder leveling (HASL), the surface finish is guaranteed to have a shelf life of 12 months either sealed or not.” He added, “If left unsealed, the laminate material can absorb moisture, which can cause delamination with heat assembly reflow temperatures. The surface finish would fit into Level 2, and the laminate would fit between Levels 2 and 2a.” According to J-STD-020, Level 2 allows for one year between opening the package and use provided the boards are kept below 30°C and below 60% relative humidity. Meanwhile, Level 2a allows four weeks from opening to use.

Matt continued, “Immersion silver is limited in its shelf life and sensitivity to UV light, handling, humidity, and solvent and heat exposure, and may not be an ideal finish for PCBs that are planned for longer-term storage.” He concluded, “Sunstone recommends that any silver boards being stored to be vacuum sealed and stored in a controlled environment and would recommend Level 3 for moisture.” Level 3 indicates a much more restrictive open-to-use time of only 168 hours (seven days). For comparison, most of the components that will be placed on the PCB will be rated Level 3.

I have some old through-hole DIP 7400 series logic chips that I bought in the 1980s. I think they would be at a pretty high risk of popcorning if they were sent through a wave solder or reflow oven today. However, all is not lost if your components have been left out in the open air too long. Any reputable assembly house will be able to pre-bake the components before assembly. Pre-baking involves heating the component or PCBs to a temperature just hot enough to slowly force the moisture out, but low enough that it won’t flash boil and pop the chip. Depending on the component, baking can add anywhere from 24 hours to 72 hours to the assembly time.

Your safest bet is to either have your manufacture order all parts and PCBs for you or buy from a reputable supplier and leave everything in the original packages. Finally, if your component packages have been opened, check the original packaging or the component datasheet for moisture sensitivity and ask your assembly house to pre-bake any components that are at risk from being out in the open too long.

Duane Benson is marketing manager and CTO at Screaming Circuits.

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2019

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2018

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