Moisture Sensitivity Level And Popcorning In Assembly
A plastic packaged component absorbs water from the air, and the amount it absorbs depends on the material, the thickness of the body and the humidity it has been exposed to. When the part is heated rapidly in a reflow oven, that water turns to steam inside the package, and the pressure has to go somewhere. Understanding the moisture sensitivity level is about controlling that pressure rather than about the material alone.
This article explains what the level means, what happens to a package that has absorbed too much water, and how the control is applied on a production floor through packing, floor life and baking.
What The Level Describes
The moisture sensitivity level is a classification that states how long a part may be exposed to a defined ambient after the dry pack is opened, before it has to be baked. The levels run from one, which has no restriction, through progressively shorter floor lives, to levels that require a bake before use regardless of the exposure time.
The level is established by the manufacturer for a specific body thickness and a specific reflow profile, and it applies to the part in the condition it was shipped in. A part that is removed from its bag and left on a bench in a humid room is no longer at the level the label states, which is why the exposure has to be tracked rather than assumed.
Popcorning And Why It Happens
Popcorning is the failure that the level exists to prevent. Water absorbed into the moulding compound and into the interface between the compound and the die paddle turns to steam at reflow temperature, and the pressure builds faster than it can escape. The package then swells, and the stress either cracks the body or separates the compound from the paddle, which is a delamination.
The visible form is a blister or a crack on the underside of the package, which is why the failure is described as popcorning. A less visible form is a delamination that leaves the part working at first and failing later, because the separated interface allows moisture to reach the die. The second form is the more dangerous, because it passes both the visual inspection and the functional test.
Floor Life And Dry Packing
Dry packing is the control measure. The part is baked at the factory, sealed in a bag with a desiccant and a humidity indicator card, and shipped. The card changes colour when the internal humidity has risen, and it is the first thing to read when a bag is opened. A card that shows the highest indicator dot has been triggered means the parts may need a bake before use.
Floor life starts when the bag is opened. The part can be exposed to the factory ambient, which is defined by a temperature and a relative humidity, for the stated period, and it can be resealed with fresh desiccant to reset the clock within certain limits. The clock is cumulative in the sense that the total exposure is what matters, so a part that is left out on three separate occasions has consumed three times the time.
Baking A Moisture Sensitive Part
Baking drives the water out, and the parameters come from the manufacturer. The temperature has to be low enough not to damage the part or its terminations, which for a part on tape means the tape has to be able to survive it, and the time has to be long enough to reach the required dryness at the centre of the body. A low temperature bake takes much longer than a high temperature one, and the trade is between the time and the risk of damage.
The parts should be baked in the original packaging where the packaging allows it, or transferred to a heat resistant carrier. A bake in a tape that cannot take the temperature will deform the pockets and jam the feeder, which turns a moisture problem into a placement problem. The bake should also be recorded with the parts, because a part that has been baked repeatedly accumulates thermal exposure and its solderability deteriorates.
Delamination And Its Consequences
A delaminated interface is a path for moisture and for contamination, and it is also a mechanical discontinuity. In a part that carries a high voltage the separation reduces the dielectric withstand, and in a part that is thermally cycled the separated layers move independently, which concentrates the stress on the bond wires and the solder balls.
The failure appears later as a leakage path, an intermittent connection or a cracked joint, and it is difficult to attribute to the assembly process because the part functions when it is first tested. Where a batch of assemblies shows a rise in field failures without a change in the design, the moisture history of the components is one of the first things to review.
Control On The Production Floor
The control is a procedure rather than a component choice. Bags are opened in a controlled area, the indicator card is read and recorded, the time out of the bag is logged, and parts that exceed the floor life are baked before use. The reels that are partly used are resealed with fresh desiccant and labelled with the remaining floor life rather than with the date they were opened.
The environment matters as much as the procedure. A store room without humidity control will consume the floor life of every part in it, and a line that is shut down over a holiday with parts loaded on the machine will find those parts out of specification when it restarts. Where the climate is humid, the ambient in the assembly area should be controlled and monitored, and the soldering conditions themselves are covered in the comparison of leaded and lead free processes. Protection applied after assembly has its own rules, described for conformal coating, and the inspection that catches the resulting defects belongs to quality characteristics.
Process Control and Verification
On a design of this kind, popcorning is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.
FAQ
Can a part be used after the floor life has expired without baking? Not safely. The absorbed moisture is still present, and the reflow will produce the same internal pressure whether or not the part has been soldered before.
Does a reflow cycle dry the part out? No. It drives some moisture out and damages the part while doing so. The dry pack exists because the reflow itself is the event that the moisture makes dangerous.
Why does the failure sometimes appear to be a joint problem? Because a delamination changes the mechanical path from the die to the board, so the solder joint carries a load it was not designed for. The root cause is in the package rather than in the joint.
