PCBA Packaging Cushioning: Shock, ESD and Moisture Barrier Design
Assembled boards leave the factory with more value per kilogram than almost anything else in the shipment, and pcba packaging is the only thing standing between a finished product and the handling it will meet on the way to the customer. Cushioning, static protection and moisture control are three separate requirements that have to be solved together.
What Has to Be Protected and From What
A populated board is vulnerable to four things in transit: mechanical shock, vibration, electrostatic discharge and moisture. Shock and vibration damage solder joints, large components and connector bodies; static discharge damages semiconductors; moisture corrodes terminations and creates ionic paths that later fail in the field. A package that solves one of these and ignores the others is not a protected package.
The protection level should follow the product rather than the shipping method. A board with tall electrolytic capacitors and heavy transformers is fragile in a different way from a board with a fine pitch BGA and no tall parts, and the cushioning that suits one will not suit the other. Start from the fragility of the assembly, measured or derived from similar products, and design the package around that figure.
Cushioning Curves and How to Read Them
Cushioning material is specified by a curve that plots the peak deceleration against the static stress for a given drop height and thickness. The curve has a U shape: too little load on the foam and it does not compress enough to absorb energy, too much and it bottoms out and transmits the shock directly. The useful region is the valley, and the design problem is to place the board’s static stress inside it.
Static stress is the weight of the supported board divided by the bearing area of the cushioning, so the design controls the area. Increasing the bearing area reduces the stress and moves the operating point along the curve; increasing the thickness moves the whole curve down, reducing peak deceleration for the same stress. Read the curve at the drop height the package will actually see, because a curve quoted at 76 cm is not valid at 122 cm. Cushioning also has a usable thickness limit. Beyond a point, additional thickness adds volume without reducing the peak acceleration, because the material has already absorbed the available energy. Where the curve shows that the required thickness is impractical, the answer is a different material or a larger bearing area, not a thicker block.
Fragility, Drop Height and Package Mass
Fragility is expressed as the maximum acceleration the product can survive, usually in multiples of gravity. A robust product might be rated at 100 g, a delicate assembly at 25 g. Drop height comes from the distribution environment and from the mass of the package: heavier packages are handled more carefully in some operations and less carefully in others, and the standard practice is to test to a height that corresponds to the package mass rather than to the shipping mode alone.
The relationship between package mass and drop height is the reason a heavier board does not automatically need more cushioning. A heavy board in a large carton generates more energy on impact, but it also compresses the foam further, which is why the static stress matters more than the weight alone. Work the numbers with the actual bearing area rather than assuming that a heavier product needs a proportionally thicker cushion. Re verify whenever the board or the carton changes. Adding a heat sink raises the mass and can raise the centre of gravity, which changes the orientation the package is likely to land on and therefore the drop orientation that has to be tested.
Static Load, Creep and Long Term Storage
Cushioning materials creep under a static load. A foam that performs well in a drop test will slowly take a set if it is left under load for months, and the board will no longer be held in the position the design assumed. Creep is most pronounced in low density polyethylene foams at high ambient temperatures, and it is the reason a package that arrives intact after a week can fail after a year in a warehouse.
Design for the storage life the product will see, not only for the transit time. Where the product may sit for more than a few months, either select a material with better creep resistance or design a package in which the board is located by the carton structure rather than by a compressed foam block. Check the stack height as well: a carton at the bottom of a tall stack carries the weight of everything above it, and the cushioning under the board sees that load as an additional static stress. Temperature accelerates creep and softens many foams. Where a package will be stored in an uncontrolled warehouse in a hot climate, test the cushioning at the upper storage temperature rather than at room temperature, because the material that behaves well at 23 C may bottom out at 45 C.
Static Shielding and Dissipative Layers
Static protection has three levels, and they are not interchangeable. A dissipative material prevents charge from building up on a surface, an antistatic material limits the charge generated by contact and separation, and a shielding material prevents a discharge from reaching the board through the package. A board that is in a dissipative bag is protected from handling charge but not from a discharge to an external conductor.
The requirement is written as a surface resistance range. Dissipative materials typically sit between 10^5 and 10^11 ohms per square, shielding materials below 10^5 ohms, and the esd shielding level should be selected for what the product requires rather than for the lowest figure available. A shielding bag that is used inside another shielding bag provides no additional benefit, while a shielding bag used to protect a board that only needs dissipation adds cost and can hide a visual inspection problem. Maintain the ESD packaging with the same discipline as the ESD workstations described in our esd control notes: verify surface resistance on incoming lots, keep bags closed until the board is needed, and never reuse a bag whose inside surface has been abraded. A scratched shielding layer has a much higher resistance than its specification, and the damage is invisible.
Moisture Barrier Bags, Desiccant and Humidity Indicators
A moisture barrier bag is a different product from an ESD bag. It is a laminate with a metallised or foil layer that limits the water vapour transmission rate, and it works together with desiccant and a humidity indicator card. The bag alone slows moisture ingress; the desiccant absorbs what gets through; the card shows whether the two together were sufficient.
Size the desiccant to the bag’s interior volume and the storage duration, not to the board’s weight. The standard unit is the amount needed to keep the interior below the target relative humidity for the specified period at the specified external condition, and it scales with the exposed area of the bag rather than with the mass inside. A large bag with a small desiccant charge will fail even if the board is light. Heat seal the bag rather than folding and taping it. A taped closure leaks at the fold, and the leakage rate through a taped seam is many times that through the laminate. Verify the sealer temperature and dwell against the bag manufacturer’s specification, and inspect the seam for channels, because a seal with a wrinkle through it is an open path.
Board Level Protection Inside the Package
Inside the bag, the board still needs protection from itself. Connectors, tall components and stiffener bars are the features that will contact whatever is next to the board, and a stack of boards separated only by a thin sheet will suffer connector damage and solder mask abrasion. Use a separator with enough thickness to clear the tallest component and locate the boards so that they cannot move laterally.
Where the boards are packed vertically in a slot, the retention has to prevent both sliding and rotation without pressing on components. A slot that is slightly too wide lets the board rock and abrade the edges; one that is too narrow compresses the board and adds a bending load that combines with creep and temperature. Check the retention over the storage temperature range, because the coefficient of thermal expansion of a plastic carrier is much larger than that of a warpage prone laminate. For boards that carry a shielding frame or a metal stiffener, add protection for the exposed metal edges too. They score the bag from the inside, and a scored shielding layer becomes a path for moisture as well as a loss of static protection.
Drop Testing and Package Qualification
A package is qualified by test, not by calculation. The standard sequence is a drop test on a defined number of cartons from the specified height in the specified orientations, followed by a visual inspection and a functional test of the boards inside. The test should be performed on a package carrying real boards or a mass and centre of gravity equivalent, because a dummy that is too light will not deform the cushioning in the same way.
Test the orientations that the package is likely to land on, and include the edge and the corner as well as the flat face. Corner drops concentrate the energy and are usually the worst case for a board that is supported at its centre. Where the package will be stacked, add a compression test at the stacking load and the storage temperature, and hold it for the storage duration rather than for the duration of the test. Keep the test boards and inspect them, not only the carton. A package that arrives visually perfect can still have transmitted enough acceleration to crack a solder joint on a heavy component, and the crack will not show until the unit is thermally cycled. Include a functional test and, for high value products, a sample cross section of the joints most likely to be affected.
Labelling, Handling and Receiving Inspection
The label carries information that the package cannot. Mark the moisture sensitive level if the assembly contains a moisture sensitive device, mark the ESD susceptibility, and mark the orientation and the maximum stack height. Where the package has been opened and resealed, the label should record the date and the remaining floor life, so that the receiving inspection knows whether the bag needs replacement desiccant.
Receiving inspection should verify the package before the product. Check the humidity indicator card, look for a broken seal or a compressed cushion, and confirm that the carton has not been dropped hard enough to crush the corners. A crushed corner is evidence of an impact far above the drop test height, and the boards inside should be inspected rather than accepted on the basis that the outer carton appears intact.
FAQ
How much cushioning does a PCBA package need? Read it from the cushioning curve for the material at the drop height the package will see, and place the board’s static stress, which is its weight divided by the bearing area, inside the valley of the curve. Add thickness only while it lowers the peak acceleration; the right answer is often a larger bearing area rather than a thicker block.
Is a moisture barrier bag the same as an ESD bag? No. An ESD bag controls static charge, either by dissipating it or by shielding the contents, while a moisture barrier bag limits water vapour transmission. A board that needs both requires a bag that does both, together with desiccant sized to the bag volume and a humidity indicator card.
What has to be checked when a package arrives? Check the humidity indicator card, the seal integrity and the cushioning condition before opening the boards. A crushed corner or a card showing the 20 percent spot fully pink means the goods experienced more stress than the design assumed, and the boards should be inspected rather than accepted on appearance.
