A plastic IC package absorbs moisture from the air, and when reflow heat flashes that moisture to steam the package can crack — the “popcorn” failure. The moisture sensitivity level (MSL) rates how long a part survives outside its sealed bag before that risk sets in. This guide explains the MSL 1–6 scale, the floor-life clock, when to bake, and the reflow-temperature ceiling that keeps the rating valid.
Key takeaways
- MSL is a 1–6 scale from IPC/JEDEC J-STD-020; MSL 1 is unlimited floor life, MSL 6 needs a bake before every reflow.
- Floor life is the time a part can sit in open factory air (≤30 °C/60 % RH) before reflow or a bake is required — MSL 3 is 168 hours, MSL 4 is 72.
- J-STD-020 classifies the part; J-STD-033 governs how everyone downstream handles it.
- Baking resets the clock; a dry cabinet only pauses it. A tripped humidity-indicator card forces a bake regardless of remaining floor life.
- Bake time is set by package thickness, not MSL — 125 °C for 24 h at ≤1.4 mm, up to 192 h at ≥4.5 mm.
- Your reflow peak must not exceed the part’s classification temperature (Tc) — 260 °C for thin packages, dropping to 245 °C for thick, high-volume bodies.
What MSL is and why it exists
What does MSL mean for a component? MSL is the moisture sensitivity level, a rating from IPC/JEDEC J-STD-020 that tells you how vulnerable a non-hermetic (plastic) surface-mount package is to moisture-induced damage during reflow soldering. It defines how long the part can be exposed to ambient air before it must be soldered or dried.
The failure mechanism drives everything. Plastic mold compound is permeable, so moisture slowly diffuses in and collects at internal interfaces. During reflow the package hits 240–260 °C, that trapped water flashes to steam, and the vapor pressure can delaminate the die from the leadframe, lift bond wires, crack the mold, or — in the worst case — audibly split the package. J-STD-020 was written to classify that risk so it can be managed instead of discovered in the field.
The MSL levels and floor life
MSL runs from 1 (least sensitive) to 6 (most sensitive), with the floor life measured at the reference condition of ≤30 °C and 60 % relative humidity. Floor life is the cumulative time the part may spend outside its moisture barrier bag before reflow must occur or a bake is required.
| MSL | Floor life (≤30 °C/60 % RH) | Dry pack? |
| 1 | Unlimited (≤30 °C/85 % RH) | No |
| 2 | 1 year | Yes |
| 2a | 4 weeks | Yes |
| 3 | 168 hours | Yes |
| 4 | 72 hours | Yes |
| 5 | 48 hours | Yes |
| 5a | 24 hours | Yes |
| 6 | Time on label; bake before use | Yes |
Source: IPC/JEDEC J-STD-020 levels; Microchip and partsbox MSL references.
Every level except MSL 1 ships dry-packed in a moisture barrier bag with desiccant and a humidity indicator card, because once the bag is opened the clock starts. Fine-pitch BGAs, thin QFNs, and large packages commonly land at MSL 3, which is why the 168-hour window is the number most receiving docks track.
J-STD-020 vs J-STD-033 — who owns what
Two standards govern moisture, and confusing them causes real handling errors. J-STD-020 is the laboratory classification: the IC vendor soaks a virgin package under defined humidity, runs it through a reflow profile, and assigns the MSL. That is the vendor’s responsibility — Texas Instruments, Infineon, ST, NXP.
J-STD-033 is the downstream handling standard: maximum humidity in the dry bag, the HIC thresholds that trigger a re-bake, the bake recipes that reset accumulated exposure, and the warning label on every reel. It is owned by everyone after the factory — distributor, broker, freight forwarder, and EMS assembler. The consequence is blunt: a part qualified to MSL 3 only behaves as MSL 3 if J-STD-033 handling has been unbroken from the factory dry-pack to the reflow oven.
The floor-life clock: pause, reset, and the HIC override
J-STD-033 treats floor life as a cumulative clock, and there are only two ways to manage it. Baking resets the clock to zero. Storing in a dry cabinet (<5 % RH) pauses it — the time stops counting but does not roll back. Everything else adds to accumulated exposure.
The humidity indicator card is the override that catches people out. It sits inside the bag and changes color at set RH thresholds, typically 5 %, 10 %, and 60 %. Standard EMS practice is that when the 10 % dot has turned pink, the contents are considered exposed and must be baked before reflow — regardless of how much floor life the paperwork says remains. If you open a bag and cannot prove the exposure history, the working rule across the industry is simple: if you don’t know, bake.
Baking: recipe set by geometry, not MSL level
The most common misconception is that a higher MSL means a longer bake. It does not. Bake time is set by package body thickness, because thicker mold compound means a longer moisture-diffusion path. The standard high-temperature recipe from J-STD-033 is 125 °C, with duration scaling by thickness.
| Body thickness | Bake at 125 °C |
| ≤1.4 mm | 24 hours |
| 1.4–2.0 mm | 48 hours |
| 2.0–4.5 mm | 96 hours |
| ≥4.5 mm | 192 hours |
Source: J-STD-033 Table 4-1 (125 °C column), as reproduced by TI and Infineon.
An MSL 5a thin QFN and an MSL 3 thin QFN of the same thickness both take 24 hours — the MSL level does not change the bake time. J-STD-033 also provides lower-temperature tiers (90 °C and 40 °C) for parts or carriers that cannot survive 125 °C, but those take far longer. Two practical constraints: parts in tape, tube, or standard JEDEC trays must be moved to bake-rated carriers before a 125 °C bake, and repeated high-temperature bakes can degrade solderability, so bake only when the clock or HIC requires it.
The reflow ceiling: classification temperature (Tc)
MSL classification is tied to a specific peak reflow temperature, the classification temperature (Tc), measured on top of the package body. The vendor qualifies the part up to Tc; in production, your reflow peak (Tp) must not exceed it. Push hotter and the MSL rating no longer applies.
For lead-free assembly, Tc depends on package thickness and plastic volume:
| Package thickness | Vol <350 mm³ | Vol 350–2000 mm³ | Vol >2000 mm³ |
| <1.6 mm | 260 °C | 260 °C | 260 °C |
| 1.6–2.5 mm | 260 °C | 250 °C | 245 °C |
| >2.5 mm | 260 °C | 245 °C | 245 °C |
Source: IPC/JEDEC J-STD-020 Pb-free classification temperatures (Table 4-2).
Thin packages tolerate the full 260 °C; large, thick, high-volume bodies are limited to 245 °C because their thermal mass and larger moisture reservoir raise the popcorn risk. A related J-STD-020 rule: a Pb-free part must be reworkable at 260 °C within 8 hours of removal from dry storage or bake, so plan rework inside that window.
Handling workflow: receiving to reflow
A clean MSD process follows the same path every time:
- Receive the reel and read the MBB label for MSL, Tc, and seal date.
- Check the HIC on opening; if the 10 % dot is pink, bake before use.
- Start the floor-life clock at bag-open, logged against the level’s hour limit.
- Store unused parts in a <5 % RH dry cabinet to pause the clock, or reseal with fresh desiccant.
- Bake at 125 °C for the thickness-based duration if the clock expires or the HIC trips, using bake-rated carriers.
- Reflow with a peak below the part’s Tc, and treat the finished board as the MSL of its most sensitive component.
Common MSL mistakes
The failures cluster in a few places. Leaving an MSL 3 reel out over a long weekend blows the 168-hour window — a leading cause of popcorn defects that surface months later as intermittent joints. Trusting the floor-life paperwork while ignoring a tripped HIC is another; the card overrides the clock. Assuming a dry cabinet resets the clock is a third — it only pauses it. And running a hot reflow profile past a thick package’s 245 °C Tc voids the rating even if the part never left its bag. Each is preventable with the label, the card, and the two tables above.
Frequently asked questions
What is the floor life of MSL 3?
An MSL 3 component has a 168-hour floor life at the reference condition of ≤30 °C and 60 % RH — the cumulative time it can spend outside its moisture barrier bag before reflow must occur or the part must be baked. Higher temperature or humidity shortens that window.
When do you need to bake components?
Bake when the floor-life clock has expired, when the humidity indicator card’s 10 % dot has turned pink, or when you cannot prove the exposure history. The standard recipe is 125 °C for a duration set by package thickness, from 24 hours for thin parts up to 192 hours for thick ones.
What is the difference between J-STD-020 and J-STD-033?
J-STD-020 is the vendor’s laboratory classification that assigns the MSL. J-STD-033 is the downstream handling standard — floor life, dry-pack, HIC thresholds, and bake recipes — followed by distributors and assemblers. The MSL rating only holds if J-STD-033 handling is maintained end to end.
What is popcorning?
Popcorning is package cracking caused by absorbed moisture flashing to steam during reflow. The vapor pressure delaminates internal interfaces and can split the package audibly, like popcorn. It is exactly what the MSL system exists to prevent.
Does a dry cabinet reset the floor-life clock?
No. A dry cabinet below 5 % RH pauses the clock — time stops accumulating — but it does not roll it back. Only a proper bake per J-STD-033 resets accumulated floor life to zero.
Bottom line
Treat the MSL rating as a handling contract, not a label. Read it and the Tc off the bag, start the clock at bag-open, and manage it with the two levers that actually work — bake to reset, dry cabinet to pause. Respect the HIC override, bake by package thickness rather than MSL level, and keep your reflow peak under the part’s Tc. Do that and the moisture never becomes the reason a good board fails months after it shipped.
