April 1, 2006
In addition to dexterity, moisture is a key ingredient of any successful magic trick. In fact, many magicians moisturize their fingertips to improve their ability to perform sleight-of-hand tricks involving cards, coins and other objects.
However, moisture is an invisible pest in the magical world of electronics manufacturing. Packages made from plastic are susceptible to moisture-related failure during board assembly. Excess moisture can turn to steam during reflow, creating defects and reducing production yields.
Electronic devices can suffer internal damage if they are not handled and stored according to industry standards. Ball grid array (BGA) and chip-scale packages are especially sensitive to moisture, and damage to these components can be difficult to detect.
Moisture absorption and retention inside electronic packages cause numerous problems. For instance, trapped moisture can vaporize and exert internal package stresses when the device is subjected to sudden, elevated temperature, such as during reflow.
Package cracking due to such moisture-induced internal stress is called popcorn cracking. However, even if the package does not crack, interfacial delamination can occur.
Surface peeling between the die pad and the resin is caused by water vapor pressure during reflow. Surface delamination is likely, resulting in shear strain on bond wires and wire necking.
Microcracking can extend to the outside of the package. External cracking may appear on the sides, top and bottom of a component. Because the package wall is often thinnest below the die pad, bottom side cracking is the most common and very difficult to detect visually
Sensitive SideMoisture-sensitive devices (MSDs) are electronic components encapsulated with plastic compounds and other organic materials. Moisture from atmospheric humidity enters permeable packaging materials by diffusion and collects at the interfaces of dissimilar material.
During solder reflow, the combination of rapid moisture expansion and material mismatch can result in package cracking or delamination of critical interfaces within the package. Unfortunately, these internal defects are nearly impossible to detect during the PCB assembly and test process. They lead to a number of failure modes that have a negative impact on manufacturing yield and cause early failure of electronic products, such as cameras, cell phones and computers.
"The risk of failure during reflow is directly related to the concentration of moisture at the critical interface, which is near the center of the package," says Francois Monette, vice president of sales and marketing at Cogiscan Inc. (Bromont, PQ). "The maximum acceptable mois-ture content and the rate of moisture diffusion vary for each package."
According to Monette, different types of packages exhibit different sensitivity levels to moisture ingress and its effects. For instance, surface-mount packages typically absorb moisture at a faster rate than bulkier, through-hole packages. They are more prone to popcorn cracking because they are thinner and have lower fracture strength. And the reflow process exposes molding compounds to higher temperatures than through-hole soldering.
"All components that are encapsulated in plastic or that are made with organic compounds are susceptible to moisture damage," says Monette. "Large plastic BGAs are the worst, because moisture can induce additional failure modes related to component warpage, in addition to internal cracks and delaminations."
Several different factors can influence the moisture sensitivity of a package, such as the internal dimensions and design of the lead frame, the external dimension of the package, the physical properties of the die attach material and mold compound, and the reflow temperature profile.
The amount of moisture absorbed within a plastic package depends on factors such as temperature, the physical properties of the mold compound, the relative humidity of the ambient atmosphere, and how much time the component is exposed to those conditions.
The diffusion rate of moisture into the mold compound is temperature-dependent. The higher the temperature, the faster the surrounding moisture will penetrate the mold compound. The absorption process will continue until the internal moisture concentration reaches an equilibrium with the ambient relative humidity. The higher the relative humidity, the greater the amount of absorbed moisture within the plastic package.
"Moisture sensitivity is becoming a bigger problem than it used to be," claims Bjarne Moller, product manager of Valor Denmark A/S (Aarhus, Denmark), which markets an MSD traceability system. He says this trend is being fueled by multiple factors, including:
- Increased sensitivity levels due to higher reflow temperatures with lead-free solder.
- Continued reductions in package body thickness and lead pitch.
- Increased use of plastic instead of higher cost hermetic body materials.
- Higher mix production. Individual trays or reels of components take longer to empty, so parts have more time to absorb moisture.
- Transfer of manufacturing operations to extremely humid geographic areas, such as Southeast Asia.
Because electronic components will keep getting thinner in the future, Derry predicts that moisture sensitivity will continue to be a challenge. "However, there is a new MSD problem that is related to size," he points out. "If the parts are thin and over a certain length and width, then they might warp rather than crack. When that happens, the edges turn up, so the balls in the corner do not get soldered."
Moisture ClassificationTwo standards help electronics manufacturers tackle the challenges of moisture sensitivity: IPC/JEDEC J-STD-020B and IPC/JEDEC J-STD-033B. The joint industry standards were developed by IPC (Bannockburn, IL) and JEDEC Solid State Technology Association (Arlington, VA). Both standards have recently been updated to support components that may need to be processed at higher temperatures, such as devices that use lead-free solder.
The IPC/JEDEC J-STD-020B standard identifies the classification level of nonhermetic solid-state surface mount devices that are sensitive to moisture-induced stress. It is used to determine what classification level should be used for initial reliability qualification. Once identified, devices can be correctly packaged, stored and handled to avoid subsequent thermal and mechanical damage during reflow or repair.
The IPC/JEDEC J-STD-033B standard provides manufacturers with standardized methods for handling, packing, shipping and using moisture-sensitive surface-mount devices. It defines methods and outlines procedures to use to avoid damage from moisture absorption and exposure to reflow temperatures that can result in yield and reliability degradation. The standard helps end users achieve safe and damage-free reflow with the dry packing process.
Both of the industry standards classify moisture sensitivity levels (MSL). Each level is expressed numerically, with the MSL number increasing with the vulnerability of the package to popcorn cracking. For instance, MSL 1 refers to electronic devices that are immune to popcorn cracking regardless of exposure to moisture. On the other hand, MSL 6 devices are most prone to moisture-induced fracture and have an extremely short floor life. The floor life of a part is the amount of time that it can be exposed to the environment and still be considered safe to reflow.
"We see many more parts that are level 4, 5 and 6 today than we did a few years ago," says Monette. "Many technology trends in electronic packaging make newer generations of components more sensitive to moisture, such as smaller, thinner packages and bigger dies. That makes those components less robust. They can be damaged with a lower moisture content, which means their maximum floor life is shorter."
While the classifications provide uniform standards, they also contribute to some misperceptions about moisture sensitivity. "The most common misunderstanding is about how the level of humidity can affect the floor life," notes Derry. "For example, it is said that a level 4 part has a floor life of 72 hours. But, that 72 hours is at the default conditions of
The Lead-Free ChallengeAs more and more manufacturers shift to lead-free alloys, moisture control becomes a bigger challenge. This summer, lead must be eliminated from electronic devices produced in or imported to the European Union. The Restriction of Hazardous Substances (RoHS) directive takes effect on July 1.
China is taking a similar lead-free stance, while various bills have been proposed in California and Canada. The goal of the regulations is to restrict the use of hazardous substances and encourage widespread recycling of electronic components.
The effort to go lead-free is driven by a combination of environmental considerations, government legislation, and the marketing advantages of lead-free electronic packages. Some leading manufacturers have already voluntarily eliminated lead from their products, while other are trying to catch up with the impending legislation.
The lead-free initiative is significantly affecting the moisture sensitivity issue, because of the higher reflow temperature associated with lead-free reflow. With lead-free solder, efficient temperature control is extremely important. In addition, high processing temperature requires 15 percent to 25 percent more energy, which makes some electronic assemblies prone to warping and other problems.
"Understanding the physical and chemical properties of the lead-free solder alloy is important, since many have reduced wetting behavior and higher surface tension," says Peter Biocca, senior market development engineer at the Kester div. of Northrop Grumman Corp. (Des Plaines, IL). "Higher thermal profiles with lead-free may require component requalification to new moisture sensitivity limits. This needs to be known and adequate measures taken to avoid moisture issues such as popcorning, delamination and cracking during lead-free reflow."
In a tin-lead soldering process, reflow temperatures range from 220 to 225 C. However, in a lead-free process, the typical reflow temperature ranges from 245 to 260 C.
"The effect that is taking place inside a component during reflow is similar to heating water in a closed vessel past the boiling point," explains Derry. "As the temperature increases, the pressure increases. And if the internal pressure is double, we can expect a corresponding increase in the frequency and magnitude of damage. I have seen figures that show that water at lead-free process temperatures causes a pressure that is roughly double the pressure at tin-lead process temperatures."
"The higher temperature increases the water pressure inside the components, and as a result, the allowable moisture content and associated floor life have to be reduced," adds Cogiscan's Monette. "All moisture-sensitive components have to be requalified by their manufacturers for lead-free and they are typically downgraded by one or more levels of sensitivity."
An International Electronics Manufacturing Initiative Inc. (Herndon, VA) project examining the impact of lead-free processing temperatures on many existing components showed that MSL ratings on these components degraded when the reflow or processing temperatures increased.
"For every 10 degree increase of temperature, it was observed that the MSL rating degraded by one level," says Michelle Ogihara, sales and marketing coordinator at Seika Machinery Inc. (Torrance, CA). "Some failures observed included microcracking, delamination, and even a breakdown in component packaging materials.
"If nothing changes in the materials that make up the current components, MSL ratings could increase up to three levels should there be a 30 degree increase in process temperatures," warns Ogihara. "This would significantly affect how components are handled, processed and stored. Suddenly, manufacturers that are currently handling MSL 2 or MSL 3 components are now looking at processing MSL 5 or MSL 6."
Dry SolutionsAs the electronics industry experiences an increasing level of failures related to moisture sensitivity, manufacturers are investing more time and money to safeguard their assembly lines. In fact, some observers claim the effort is similar to the battle mounted against electrostatic discharge (ESD) 10 years ago.
Engineers have several ways to combat moisture. Dry pack is a process that uses an oven to bake components and drive all moisture out of the package. Devices are then heat- or vacuum-sealed in a waterproof bag to prevent any subsequent absorption of moisture.
A moisture barrier bag restricts the transmission of water vapor and is used to pack moisture-sensitive devices. A humidity indicator card is packed inside the bag, along with a desiccant, to aid in determining the level of moisture to which electronic devices have been subjected during shipment.
A humidity indicator card features three vertical color spots that are sensitive to relative humidity values of 5 percent, 10 percent and 60 percent. The card changes color-typically from blue (dry) to pink (wet)-when the indicated relative humidity is exceeded.
Dessicant is an absorbent material, such as silica gel or zeolite, that's used to maintain a low relative humidity inside a bag or carrier. The amount of dessicant used is calculated according to the bag surface area and water vapor transmission rate in grams per 100 square inches in 24 hours, to maintain an interior relative humidity of less than 10 percent at 25 C.
Dry boxes are an alternative to moisture barrier bags that can greatly reduce the risk of operator handling errors. The metal cabinets feature tightly sealed doors and multiple shelves for storing trays and reels for extended periods of time. A good dry box maintains relative humidity at 5 percent or less for indefinite safe storage, as stated in the guidelines under the IPC/JEDEC J-STD-033B specification.
Humidity is removed from the cabinets by use of a powerful dessicant. Moisture absorbed by the dessicant is vaporized and released outside the dry box. Digital controls record the temperature and humidity over time.
Nitrogen cabinets are a more expensive way to purge moisture. They use a constant positive pressure flow to void the cabinet of all oxygen, which forces out all moisture and contamination.
Several companies have developed automated control systems that allow electronic manufacturers to manage and track their MSD devices. They use automatic data collection technology, such as bar coding and radio frequency identification. The automated systems eliminate the need for manual procedures, such as identifying MSDs, filling out log sheets and entering time calculations, which are time-consuming and open to human error.
Monette says his company has seen an increasing level of interest for its control system since it was unveiled several years ago. Cogiscan's system provides real-time tracking of all MSDs. "It provides precise exposure time tracking for each individual tray or reel containing moisture-sensitive components from the time they are removed from the protective dry bag through placement and reflow," claims Monette.
Valor also markets a traceability system that helps ensure that MSDs will be safe during reflow. "Failure analysis techniques can identify that the root cause of failure was excessive moisture prior to reflow," says Moller. "However, it is impossible to establish at what point the component was overexposed.
"This might have occurred during the assembly process, but the components may also have been exposed earlier in the supply chain, such as when the parts were sent to an outside vendor for IC programming," adds Moller. "In such cases, the only option for the assembler is to demonstrate that within its own facility the components were handled according to industry standards. This will at least indicate that root cause was possibly elsewhere in the supply chain."
Despite those efforts, most experts believe there's a need for more industrywide education. The Surface Mount Technology Association (Edina, MN) created an MSD Council, which aims to promote the practice of moisture-sensitive devices control in electronic assembly processes.
However, there are still several misunderstandings and misconceptions related to moisture sensitivity, says Monette, who serves on the council. "In general, the industry still needs a lot more education on this topic," he points out. "Once engineers really understand the issue, they can start to implement effective programs.
"This is not a simple matter that can be resolved by installing a few dry cabinets and a bake oven," warns Monette. "People have to understand that the effort required to bring this issue under control is on a similar level to what has been done with ESD control in the past."
One of the biggest misperceptions about moisture sensitivity is related to the fact that it's hidden from view. Indeed, it typically isn't a defect that can be clearly distinguished immediately after assembly. Therefore, there's a natural tendency to think that it's not a serious problem. Sometimes, problems due to moisture-sensitivity are more obvious, such as components lifting off of a board after reflow.
"Many MSD defects, such as microcracking or popcorning, are not easily identifiable because they occur under the packages or must be seen with a magnifying scope during inspection," says Seika Machinery's Ogihara. "Also, returns from the field may or may not be attributable to a problem with moisture sensitivity, as some assemblers may chalk it up to some other process-related issue." A