When SMT parts that sat open need baking before reflow

The kit for a small build arrives with a reel that was opened months ago, a cut strip in a zip bag, and a tray of BGAs with the humidity card missing. Everything is the right part number. Nothing on the label says whether it is still safe to put through a reflow oven. The person who opened the bag did nothing wrong on purpose, and the damage, if it happens, will not show up until the board is in the oven or, worse, in the field.

This comes up constantly on distributor forums and assembly boards: “my parts have been sitting out, do I really have to bake them?” The short answer is that the part’s moisture sensitivity rating decides, not your judgment. This note explains what the rating means, how to read what came in the bag, and what to do when the clock has run out.

Why a plastic package cares about humidity

Molded plastic packages are not sealed. Over days to weeks they absorb water vapor from the air, and the resin, the die attach, and the leadframe or substrate all hold some of it. That water is harmless at room temperature. During reflow the package body passes well above the boiling point of water, and moisture trapped inside turns to steam faster than it can diffuse out. The pressure cracks the mold compound, delaminates the die from the paddle, or lifts bond wires. The industry name is popcorning, because a badly affected package can audibly crack and visibly bulge in the oven. DigiKey’s short explainer on protecting surface-mount parts from moisture damage describes the effect and why it is a reflow problem rather than a storage problem.

The insidious part is that most popcorned packages do not look damaged. Internal delamination shows up as an intermittent bond, a part that passes test at room temperature and fails hot, or a field return a year later. That is why the rules are written as hard limits rather than as advice.

Reading the rating and the clock

Every non-hermetic surface-mount package carries a Moisture Sensitivity Level, MSL, set by the manufacturer under the joint IPC and JEDEC classification standards. The level tells you how long a part can sit outside its sealed bag, at a stated temperature and humidity, before it must be baked. Renesas publishes a compact version of the table in its packing and handling guide, and the levels are the same across vendors because they come from the same standard.

Level	Floor life out of the bag
1	Unlimited
2	One year
2a	Four weeks
3	168 hours
4	72 hours
5	48 hours
5a	24 hours
6	Bake before use, time on the label

Those durations assume a factory floor at or below the conditions printed on the label, which for levels 2 through 6 is generally 30 °C and 60 percent relative humidity. A warmer or damper room shortens the real window. The clock is cumulative: it starts when the bag is opened, pauses only while the parts sit in dry storage or a resealed bag with fresh desiccant, and does not reset because the parts went back in a drawer.

Three things arrive in a proper dry pack and each answers a question:

• The bag label carries the MSL and the seal date. If the label is gone, look the rating up on the manufacturer’s product page. TI, for instance, points customers to the MSL information on the bag or box and to the quality section of each device’s product folder.
• The humidity indicator card tells you whether the bag stayed dry. Vendors set their own reading rule. TI’s shelf-life FAQ says parts need baking if the card shows pink at the 10 percent indication. Renesas describes its card as saturated when the 30 percent spot has turned lavender. Read the rule that came with that bag, not a rule remembered from another vendor.
• The desiccant only works while the bag is sealed. A bag that has been opened and folded shut is not dry storage.

The buyer-side traps are predictable. A reel bought last year and opened for a prototype run is now an unknown. A cut strip from a broker or a marketplace seller arrived with no bag, no card, and no history, so it should be treated as if the floor life has been exceeded. MSL 1 parts, which include most passives and many small discretes, are the exception and can sit on the shelf indefinitely. The parts that matter are the ones people worry about least: a QFN microcontroller, a BGA memory, a module with a plastic shield, an LED, an inductor with a molded body, a power module with an exposed pad.

What a bake does and does not fix

Baking drives the absorbed moisture back out slowly, at a temperature low enough that the water leaves by diffusion instead of steam pressure. Once dry, the floor life clock starts again from zero. Renesas gives the general case as up to 24 hours at 125 °C in a heat-resistant tray, and warns that going hotter risks lead corrosion from outgassing. Thicker packages and higher levels take longer, and the vendor’s document for the specific package is the reference, not a generic table.

The constraint that surprises small shops is the packaging. Renesas states plainly that tape and reel is not heatproof and must not be baked, and that magazines and non-heatproof trays are the same. Baking a reel at the standard temperature warps the carrier and lifts the cover tape, and the reel that comes out will not feed. The alternatives are to transfer parts to a heat-resistant tray and bake at the higher temperature, or use the much longer low-temperature bake that some vendors describe for parts left in tape. Either way, parts that were on a reel come off the pick-and-place feeder path and go back on by hand, which is where mistakes in orientation and count creep in.

A bake also has costs the rating does not mention. Repeated or prolonged heat oxidizes terminations and can degrade solderability, so a part baked several times over its life is not equivalent to a fresh one. Ink marking can fade. And a bake does nothing for parts that were already reflowed once with moisture inside, because the damage is mechanical and permanent.

What a careful engineer checks before the kit goes in the oven

• Every line of the BOM has a known MSL. The datasheet or product page states it. If the part is level 3 or higher, it is on the watch list.
• Every bag on the watch list has a label, a card, and a story. Seal date, open date, hours out. If any of those is missing, the part is assumed expired.
• The card is read against the vendor’s rule for that card, at room temperature, immediately on opening.
• Reeled parts that need a bake are planned as a tray bake, with time in the schedule for de-reeling and re-reeling, or the build is re-quoted with fresh stock.
• Kitted parts from consignment carry the same rules as purchased ones. A customer who sends a half-used reel is sending an unknown, and it is better to say so before the build than after.
• Second reflows count. A double-sided board that goes through the oven twice, or a rework station, exposes the same package to the same steam mechanism. Floor life covers the interval between opening the bag and the last high-temperature step, not the first.

The design side can reduce the exposure too. Choosing a lower-MSL package variant where the vendor offers one, avoiding a level 5 or 6 part for a product that will be built in small batches over years, and consolidating a build so opened reels are consumed rather than stored all lower the odds that a bake is ever needed.

We treat moisture sensitivity as a kit-inspection step rather than an oven step, because by the time a package is in the oven the decision has already been made. If you are sending parts for a small assembly run, tell us when each bag was opened and include the humidity card, and we will tell you before the build which parts are fine, which need a bake, and which are safer replaced.

Sources: Renesas packing and handling guide, TI product shelf life, TI product shelf life FAQs, DigiKey on moisture damage to surface-mount parts, Moisture sensitivity level overview.