Moisture Sensitivity Level and Floor Life Control

A plastic package absorbs moisture from the air. When it is heated to reflow temperature the moisture turns to steam, and the pressure can crack the package from the inside, delaminate the die attach or lift the wire bonds. The damage is called popcorning, and it can be invisible from the outside. Moisture sensitivity level and floor life are the two numbers that define how long a part can be exposed before it must be dried.

What the Two Numbers Mean

The moisture sensitivity level classifies the package by the amount of moisture it can absorb and still survive a reflow cycle after a defined exposure. A lower level number means the package is more sensitive and must be handled more carefully, while a package with a high level can be exposed for longer.

The floor life is the time the part may be exposed to ambient conditions after the dry pack is opened, before it must be baked. It applies from the moment the bag is opened, not from the moment the part is placed, and it accumulates across every exposure. A part that is removed, left on the bench and returned has consumed floor life whether or not it was heated.

Dry Pack, Desiccant and the Indicator Card

The dry pack is the control. A moisture barrier bag with a desiccant absorbs the residual moisture and keeps the internal humidity low, and a humidity indicator card shows whether the bag has remained sealed. A card that shows a high reading, or a bag that is damaged or has been open for an unknown time, means the parts cannot be treated as dry.

The bag should be opened only when the parts are needed, and the parts should be used from the bag rather than transferred to an open container. Where a partial reel is returned, it should be resealed with fresh desiccant or placed in a dry cabinet, because a bag that is left open offers no protection and the floor life continues to run.

Tracking Exposure in Practice

Exposure has to be recorded, because it cannot be estimated. A label on the reel with the date the bag was opened, plus a running total of the hours outside the dry environment, is the simplest system that works. Where the parts go through several operations, each one has to add to the total rather than reset it.

The tracking is usually managed by the machine or by the warehouse system where one is available, which removes the reliance on the operator memory. Where it is manual, the label should be checked at the point of loading rather than at the end of the shift, because a reel that has exceeded its floor life has to be baked before it is placed.

Baking and Its Limits

Baking removes the absorbed moisture by holding the parts at an elevated temperature for a defined time. The temperature and the time come from the component supplier, and both matter: a temperature that is too low does not remove the moisture from the interior, and one that is too high can damage the terminations or the packaging.

Baking also has a limit on the number of cycles. Repeated baking oxidises the terminations, which reduces solderability, and the supplier specification usually states how many times a part may be baked. Where a part has exceeded its allowed bakes, the solderability has to be assessed before it is used. The solderability checks are the same ones described in solder defect assessment.

Popcorning and Its Symptoms

Popcorning produces several damage modes. A package can crack on the underside where the crack is invisible, the die attach can delaminate, the wire bonds can lift, and the package can bulge. The electrical result varies from no effect at all to an immediate open circuit.

The visible sign is often a small crack or a bulge after reflow, and a cross section reveals the internal separation. Because the crack is on the underside of the package, an external inspection is not reliable, so the control has to be the moisture management rather than the inspection.

Re-reflow and Double Sided Assemblies

A part that survives the first reflow may fail during the second, because the moisture absorbed between the two cycles adds to the load. A double sided assembly and any process with more than one thermal cycle should therefore treat the floor life requirement as more demanding than a single pass product.

The design review should consider the number of thermal cycles the assembly will see, including rework. A part that is marginal for one reflow may be unsuitable for a product that will be reflowed twice and reworked once.

Process Controls and Records

The controls are straightforward: keep the parts in the dry pack until needed, record the exposure, bake when the floor life is exceeded, respect the bake limits and control the storage environment. Each of those is a procedure rather than a technology, and the failure comes from the procedure being skipped rather than from a lack of capability.

The gopcb materials team labels every moisture sensitive reel with its level, its floor life and the date the bag was opened, and the label travels with the reel through the line. Where a reel has to be baked, the bake is recorded with the part number and the length of the bake so that the number of cycles is known, and the record is available when a package failure is investigated.

Checks Before Release

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

Where the supplier and the user both measure the same property, they should agree on the method before the first delivery.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Does floor life reset when the part is baked? Yes, after a full bake cycle, but the number of bakes is limited. The part is dry again but its terminations have been exposed.

Can a moisture sensitive part be used without baking if it looks dry? No. The absorbed moisture is inside the package and cannot be seen.

Does a humidity indicator card need to be checked when the bag is opened? Yes. A card that indicates moisture means the parts must be baked before use.