Moisture Sensitivity Levels and Baking Before Assembly
Plastic packages absorb water from the air, and that water turns to steam the moment the part enters a reflow oven. What follows can be a crack, a delaminated die attach, or a bond wire that breaks months later in the field. Moisture sensitivity levels exist to manage that risk through storage, handling, and baking rules that are easy to follow once you understand what they protect.
What Moisture Sensitivity Levels Describe
A moisture sensitivity level states how long a package may be exposed to factory air before it must be dried again. The scale runs from MSL 1, which needs no special handling, to MSL 6, which requires baking immediately before use. The classification is determined by the package body, the mould compound, the die attach, and the internal volume of the part.
Smaller packages generally tolerate less exposure, not more. A thin, fine-pitch body absorbs moisture quickly and has little material to resist the pressure that forms inside. Designers who default to a low-profile package for space reasons should confirm its moisture sensitivity rating early, because an exotic part with a difficult rating can add handling cost to the whole build.
Why Trapped Moisture Causes Popcorning
Water absorbed into the mould compound flashes to steam above 100 degrees Celsius and expands rapidly. If the vapour cannot escape fast enough, it creates pressure at the weakest interface, usually the die attach or the lead frame. The result is a visible bulge, an internal delamination, or the sharp crack that gives the failure its name.
Damage is not always visible from the outside. A package can pass visual inspection and still have a separated die surface or a broken wire bond. This is why the industry controls moisture before the part reaches the oven rather than inspecting afterwards, and why a baking step is treated as a process requirement rather than an optional precaution.
Reading the MSL Label and Floor Life
Every moisture-sensitive part arrives with a label showing the level, the floor life, and sometimes the maximum reflow temperature. Floor life is the cumulative time the part may spend out of its dry pack at a stated temperature and humidity, typically below 30 degrees Celsius and 60 percent relative humidity. Once that budget is used up, the part must be dried again.
The budget is cumulative, not per shift. Moving a reel between machines, leaving it on a feeder overnight, and returning it to the store all consume floor life. Track the time out of bag on the work order, and treat any reel whose budget is unclear as if it had been exposed, which means baking or scrapping it rather than guessing.
Dry Pack, Desiccant and Humidity Indicators
Dry packing is the standard method for shipping and storing sensitive parts. The components go into a moisture barrier bag with desiccant, the bag is evacuated and sealed, and a humidity indicator card is placed inside. The card changes colour when the internal humidity exceeds a threshold, which signals that the desiccant is exhausted or the bag has been compromised.
When a bag is opened, the parts begin to absorb moisture immediately. Take only what the shift needs, reseal the remainder with fresh desiccant, and note the date and time. A bag that has been left open, punctured, or stored in a humid area should not be treated as dry even if the card looks acceptable, because the card reflects the local atmosphere rather than the moisture already inside the packages.
When and How to Bake Components
Baking removes absorbed moisture by holding the parts at an elevated temperature for a defined time. The schedule depends on the package thickness, the body material, and the temperature the part can survive. Standard trays tolerate higher bake temperatures than tape and reel, and parts with batteries, liquid crystal displays, or certain plastics cannot be baked at all without damage.
The common mistake is baking at too high a temperature to save time. Over-baking oxidises terminations, degrades the mould compound, and can damage the carrier tape. Use the supplier’s published schedule, verify the oven’s actual temperature with a reference probe, and record the start and end times so that the parts can be traced to a specific drying cycle.
Bake Schedules for Tape, Tube and Tray
Carrier format drives the permissible temperature. Most tape and reel products are limited to around 40 to 45 degrees Celsius because the cover tape, the carrier, and the adhesive cannot take more. Trays and tubes often allow 125 degrees Celsius for a shorter period, which is far more effective at removing moisture from thick bodies but requires the parts to be transferred out of tape first.
Re-reeling after baking introduces its own risk of mechanical damage and contamination, so it should be planned rather than improvised. Where a supplier offers a high-temperature tape, that option usually pays for itself by avoiding a transfer step. Whichever route is chosen, keep the bake certificate with the reel so the exposure history stays complete.
Tracking Floor Life on the Line
On the production floor, floor life is managed with labels, timers, and simple discipline. Each reel or tray carries the time it left the bag, and the remaining budget is written where the operator can see it. When a budget expires mid-run, the correct response is to stop, reseal, and schedule a bake rather than to finish the batch and hope.
Humidity and temperature in the assembly area matter as much as the clock. A line running at high humidity consumes floor life faster than the label assumes, so monitor the room and adjust the estimate when conditions are outside the standard range. Air conditioning failures during a summer weekend have ended more than one product’s reliability.
Reflow Profile Adjustments for Sensitive Parts
A gentle profile helps moisture-sensitive parts survive. Slower ramps in the preheat region give vapour time to diffuse out of the package before the internal pressure peaks, and a slightly reduced peak temperature lowers the stress on the body. These changes must be balanced against the needs of the solder alloy and the rest of the assembly.
Profiling with a thermocouple attached to the package surface, not only to the board, shows what the part actually experiences. Some large bodies lag the board by tens of degrees, which means the oven profile and the package profile are different events. Adjust the soak and peak based on measured data, then verify the joints with X-ray or optical inspection after the first articles.
Storage, Handling and Documentation
Storage should be dry, stable, and controlled. A dry cabinet set at low relative humidity extends the practical life of opened reels and avoids repeated bake cycles, which are themselves a stress. Inventory rotation matters too, because a reel at the back of a shelf may have been opened a year ago and never recorded.
Documentation ties the whole process together. The record should show the MSL rating, the bag opening date, cumulative exposure, any bake cycles with their schedules, and the reflow profile used. When a package cracks or a device fails early, this history tells engineering whether moisture handling or something else is responsible.
FAQ
Can I skip baking if the humidity indicator card looks fine? Not if the allowable floor life has been exceeded or the bag was compromised. The card reflects the atmosphere inside the bag at the moment it is read, not the moisture already absorbed by the packages. Follow the level and the floor life rather than relying on the card alone.
Does baking damage components? It can, if the temperature is too high or the cycle is too long. Reel materials, adhesives, batteries, and some plastics have limits well below the temperatures used for trays, and repeated bakes oxidise terminations. Always use the supplier’s schedule for the specific package and carrier.
How do I know if a package has popped? External cracks are sometimes visible under magnification, but internal damage is not. Scanning acoustic microscopy detects delamination, and X-ray can reveal certain internal separations, while electrical test may pass despite a weakened joint. Prevention through storage and baking discipline is cheaper than detection.
