Ultra-Low Humidity SMT Dry Cabinet for High Tech Manufacturing
Trace moisture causes defects and failures. From wafer to IC packaging, PCB or PWB manufacturing materials, or surface mount assembly process, trace moisture will cause serious manufacturing defects and failures. These defects are results from improper storage and handling methods which could have occurred at the supplier, warehouse, shipping or the assembly process. These are especially important concerns for industries where product reliability must be tightly controlled such as automotive, defense, medical devices, aeronautic & aviation industries.
These are examples of the type of moisture related damage to a PCB assembly that can occur if incorrect storage or handling is not followed.
Image sourced from : Bob Willis [email protected] www.Bobwillis.co.uk / PCB Delamination with Bob Willis https://www.youtube.com/watch?v=pn7P-knw_lg&feature=youtu.be
Moisture in SMT / EMS
In 2013, the JEDEC Solid State Technology Association announced the newest standard of Handling, Packing, Shipping, and Use of Moisture and/or Process Sensitive Components. This document provides MSD and SMT manufacturers and users with standardized methods for handling, packing, shipping and use of moisture sensitive MSDs. Now updated to support components that may need to be processed at higher temperatures, such as lead-free processes, these methods help avoid damage from moisture absorption and exposure to solder reflow temperatures that can result in yield and reliability problems.
Eureka's Fast Super Dryer meets IPC/JEDEC J-STD-033C standards and helps achieve safe and damage-free reflow while providing maximum shelf life for components, devices and PCBs. Eureka's Fast Super Dryer (also called low humidity dry cabinet, super dryer, desiccator or humidity control cabinet, etc.) provides MSDs (moisture sensitive devices) unlimited floor life when stored at <5% RH.
Eureka’s SMT Dry Cabinets are affordable and effective solution in removing moisture in the production process, eliminating moisture related problems without using nitrogen (N2) / dry air purging. Eureka’s Fast Super Dryer dry cabinets can be used as an alternative to baking components and PCBs, eliminating heat baking problems such as breakdowns of protective coatings, lead oxidation, intermetallic growth, warping and solderability. Eureka’s ultra-low humidity cabinets provides SMD tapes, reel & tapes or reel to reels (which cannot be baked) the perfect humidity control solution. With these problems eliminated, product reliability and yield will increase.
How does moisture affect SMT production?
Moisture Control Storage for Wafer & Die
Semiconductor wafers and die are not inherently moisture sensitive. However, wire bond pads are quite sensitive and any presences of moisture may accelerate failure, such as bonding issues and delamination, which may only be discovered during product packaging. KGD (known good die) parts are typically shipped in tape and reels packaging which cannot be baked to remove moisture.
The best preventive measure is to maintain optimal controlled storage environments to prevent any failures. When in storage, die and wafers, should be stored die banks with temperature between 18°C and 24°C and relative humidity of less than 30%.
- Vacuum sealed packaging are not recommended for the long term and should be considered temporary transport packing to avoid moisture absorption and other contamination.
- Storing wafers and dies below <10%RH can result in build-up of electrostatic fields, storing over <40%RH will result in condensation and moisture ingress.
- Typically, storage in nitrogen cabinets are recommended, however, the long term cost with N2 storage is quite high.
- Desiccants could cause mechanical abrasion of particulates and the effectiveness may become harmful in the long term if excessive moisture is present.
Eureka Dry Tech’s Fast Super Dryer offers the same moisture & humidity controlled storage options with fast recovery times without using costly nitrogen (N2) purging. However, if customer has specific requirements N2 purging is available as an option.
IC Devices, Packages and Components
All semiconductor devices are susceptible to damage or degradation during storage and handling. Non-hermetic, plastic-encapsulated surface-mount devices are particularly sensitive to moisture-induced stress during printed wiring board assembly.
IPC/JEDEC J-STD-033 provides guidelines for various moistures sensitivity levels (MSL) and allowable exposure time to specific ambient temperature and moisture levels before baking is required.
- When opened packages or lots are used, the remaining devices should be resealed or placed in a dry box at no more than 10% RH within one hour of bag opening.
- If the exposure to ambient environment exceeds the allowable time, the devices must be baked before mounting. However, baking will cause oxidation and intermetallic growth causing solderability issues.
Eureka's dry cabinet utilizes powerful multi-porous molecular sieve desiccants which will provide alternative drying methods to baking oven and reduce oxidation problems caused by heating cycles.
Printed Circuit Board Manufacturing
Circuit boards if not properly dried and stored, can contain significant amounts of moisture from processing. Any moisture, even in trace amounts, will lead to delamination and warping in PCB when high reflow temperatures cause the moisture inside the layers to expand.
Until the ‘IPC-1601 printed board handling and storage guidelines’ was published in 2010, PCBs were generally overlooked for storage and moisture protection for printed circuit boards. Often times, PCBs are shipped in foil or an ESD bag instead of Moisture Barrier Bags (MBB). PCBs then arrive at the assembly floor with absorbed moisture requiring baking. IPC-1601 states that if "printed boards have absorbed excessive moisture, baking is the most practical remedy.” However, baking increases cost and cycle time and can also degradesolderability of the printed board. “In general, both the printed board fabricator and the user should strive to avoid baking by practicing effective handling, packaging, storage, and process controls…”
Baking also accelerates solid diffusion between metals, and increases intermetallic growth which leads to a “weak knee” or other solderability issues if the intermetallic layer reaches the surface and oxidizes.
Electroless nickel immersion gold (ENIG)
ENIG, immersion tin, silver, and OSP are finishes often used on printed circuit boards because they are lead-free. However, these materials are susceptible to oxidation and corrosion from exposure to moisture and humidity. This oxidation causes dewetting after soldering, which can lead to poor joints at assembly and ultimately lead to failure of the board.
Boards with these types of finishes should be stored in environments controlled between 68°F - 82°F (20°C - 25°C) with relative humidity < 50%. If not properly handle and stored, the surface finish will be adversely affected by moisture and contaminates in the air.
Epoxy Glass PREPREG
Epoxy glass prepreg contains moisture and requires specific storage environments. If prepreg absorbs excessive moisture, its bonding strength could be weakened. Long term storage of prepregs typically requires 5°C with relative humidity of 50%. Remaining unused prepregs should be stored stored flat (horizontally) in a in a cool, dry environment such as a dry cabinet—typically at 23°C or lower and 50%RH or lower, and sealed in a moisture barrier bag (MBB).
Organic Solderability Preservative Coatings
Organic solderability preservative (OSP) coatings provides solderability, ease of processing and low cost. OSP coatings are among the leading surface finish options in lead free soldering. However, compared to alternatives, OSP are most prone to oxidation due to the pure copper surface protected only by the OSP coating layer.
IPC-1601 states “Baking is NOT recommended for OSP coatings, as it deteriorates the OSP finish. If baking is deemed necessary, the use of the lowest possible temperature and dwell time is suggested as a starting point.”
Most solder pastes have a shelf life of three to six months when stored at room temperature (22°C). Unopened jars of solder paste should be refrigerated (4°C). Refrigeration will normally extend the shelf life of solder paste while protecting from the varying levels of heat and humidity often found in warehouses and offices. Allowing paste/flux temperature to rise to room temperature before opening their containers is important to avoid condensation. If the lids are removed while the paste are still cool, moisture would quickly condense on the surface of the paste/flux. As a rule, solder pastes and solder flux do not react well to moisture. Moisture-contaminated paste or flux may:
- reduced viscosity
- spatter during reflow
- produce excessive oxidation of the solder joint
Proper storage and handling of assembly components and material is one of the most important steps in total quality management. Any exposure to ambient humidity can have detrimental effects to the final product. Eureka Dry Tech provides total moisture control solutions for all services in Surface Mount and other High Tech Manufacturing services.