by Jeff Stong
At the EMPF, all military and commercial-built PCB assemblies are treated with special care. Moisture build-up in components can have a detrimental effect when reflow temperatures are applied. The proper storage of components and PCB's is essential to any assembly house. The EMPF utilizes a McDry Air Drying Cabinet by Seika Machinery, Inc.

With the increased sophistication of electronic products, moisture sensitive devices (MSD) have become widely used. MSDs present a number of challenges when used in surface mount assemblies as they may suffer internal damage during the manufacturing process if they are not handled and stored according to industry standards.

In surface mount assembly, solder paste is printed onto a PCB. Components are then mounted, and the entire board is sent through a reflow oven. In the reflow process, the entire PCB and component packages are heated along with the solder and component leads. The high reflow temperatures can cause a package containing moisture to swell and crack, affecting the performance and reliability of the component. These problems are normally the result of poor handling and storage of MSDs.

While most of the damaged components may be detected prior to shipment of the final product, many make it to the market place only to fail in the field. Although a component with external cracks may pass pre-shipment functional tests, subsequent high temperature and moisture exposure can induce the transport of ionic contaminants through these openings to the die surface, increasing the potential for failure due to corrosion. Furthermore, internal cracking and delamination can be present even if there is no evidence of external cracks. Ball grid arrays (BGAs) and chip scale packaging (CSPs) are especially sensitive to moisture and damage to these components can be very difficult to detect.

Manufacturers must pay close attention to their handling and storage of MSDs to maximize yields and to ensure the quality of their finished products.

Why do components require low humidity storage?
The plastic packaging used to manufacture surface mount devices (SMD) will absorb moisture from the atmosphere. The high temperatures involved in the vapor phase of reflow soldering can cause the absorbed moisture to expand rapidly, producting internal stress known as "Popcorning." Surface delamination is likely and this delamination results in strain on the bond wires and wire loop. Microcracking may also extend to the outside of the package.

The SMT reflow process exposes devices to higher temperatures (220oC to 245oC) than through-hole devices (135OC to 150o C). The solder reflow processes of concern are convection, Convection/IR, Infrared (IR), vapor phase and hot air rework machines.

Integrated components (ICs) are comprised of dies typically made of silicon, a die pad, which the silicon die rests on, gold or aluminum wire for the electrical connection to the leads and various plastic materials that make up the body of the component. The materials that comprise an integrated component have different thermal expansion ratios, adhesive strengths and material strength characteristics. Moisture absorbed by components vaporizes during the reflow process. The vapor pressure causes delamination and cracking in the plastic component packages. External cracking may appear on the side, top and/or the bottom of the components. As the package wall is often thinnest below the die pad, bottom side cracking is the most common and is very difficult to detect visually. External cracked components can suffer additional damage due to moisture, heat and vibration in the market place. Internally, the vapor pressure can create a void that allows the die paddle to move during temperature cycling. Surface delamination resulting from strain and breakage of the bond wires is likely.

The use of air-dry cabinets is one way of reducing moisture absorption by components. Air drying cabinets, which utilize a strong desiccant, are popular in Asia due to the extreme humidity in the region. Normally, with an air-dry cabinet, the desiccant is recycled automatically with a heating system and does not require replacement. Performance of air-dry cabinets is based on the type and the amount of desiccant used, efficiency of the desiccant recycling system and sealing of the cabinet. Two types of desiccant, silica gel and zeolite, are commonly used. Of the two, zeolite is more efficient at lower Relative Humidity (RH) levels. At 25 oC, 5% RH, zeolite can hold 20% of its weight in water compared to approximately 5.5% for silica gel.

When using an air-dry cabinet, caution must be taken to ensure that the cabinet doors are not opened too frequently or for an extended time. This will cause the RH level within the dry box to become unstable.

Improper storage of moisture sensitive components can lead to low yields and product failures in the market place. With proper handling, packaging and utilization of air-dry cabinets, manufacturers can effectively and efficiently implement proper handling procedures based on IPC/JEDEC specifications. (Reference IPC/JEDEC Standard J-STD-033). Air-drying cabinets are a good option for components that require short term storage at low RH levels. MSDs which must be used shortly after baking, can be easily staged in an air-dry cabinet. Air-dry cabinets are also convenient storage for MSDs to be used for rework. Rework technicians are able to remove the required number of components from the air-dry cabinet and are relieved from constantly having to reseal the remaining MSDs.