Humidity Indicator Card Control for MSD Storage: 6 Rules

A humidity indicator card is a small piece of paper with dots of treated salt that change colour as the air around them becomes wetter. It costs very little, it has no electronics, and it is the only evidence most workshops ever have about what happened inside a sealed bag between the day a moisture sensitive part was packed and the day the bag was opened. Yet it is also the record most often read carelessly.

The reason to care is that the card drives a decision. If the spots show that the air inside the bag stayed dry, the parts can go straight to the line and their floor life starts from the moment the bag is opened. If the spots show that the bag has taken on moisture, the parts need a bake before they can be soldered. Reading the card correctly is therefore worth more than any amount of care taken afterwards.

What the Card Actually Measures

Each dot on the card is a deliquescent salt with a defined transition point, printed so that the colour change happens at a known relative humidity. A typical card carries three spots marked five, ten and fifteen percent, or ten, twenty and thirty percent, depending on the specification it was bought to. The spot changes from blue to pink, or in a cobalt free card from brown to a lighter shade, as the humidity passes that point.

The card measures the air in the bag, which is not the same as the moisture inside the component. A part can be dry on the outside while its plastic body still holds water, and a part can be wet on the outside after an accidental exposure while the card still reads dry because the card was sealed in a dry pocket of the bag. The card is an indicator of the storage condition, not a measurement of the parts.

Reading the Spots Correctly

The card should be read at the moment the bag is opened and in the light it was designed for, because the colour difference between a changed spot and an unchanged one is not large. A spot that is only partly changed counts as changed: if the ten percent dot has turned at its edge, the bag has seen at least ten percent relative humidity somewhere in its life.

The judgement then follows the specification. Where the requirement is that the least sensitive spot still reads dry, a card with the ten percent dot fully changed is a fail and the parts require a bake. A card that is completely pink is not a borderline case, it is evidence that the bag was open, wet or damaged, and the whole batch should be treated as exposed.

Cobalt Free Cards and Colour Change

Older cards used cobalt salts, which give a crisp blue to pink transition and are now restricted in many markets. Cobalt free cards use other chemistries and often change from brown to a much paler colour, which is harder to judge under a yellow workshop light. A cobalt free card should be read against a reference card kept for the purpose, and the reference should be replaced whenever a new card batch is introduced.

The colour change is also reversible. A card that dried out again after a wet period may recover most of its colour, which is why a card that is removed from the bag and stored in the open is useless as a record of that bag. The reading has to be taken immediately on opening, before the card has a chance to dry.

Bag, Desiccant and Card as One System

The card only means something when the bag and the desiccant are right. A moisture barrier bag has to be sealed, not folded over, and the seal has to be complete across the full width. A bag that has been cut open and re-sealed with tape has no barrier value at all, whatever the card reads.

The desiccant quantity has to match the bag volume and the exposure time, and the desiccant needs to be active rather than saturated. A bag with too little desiccant will show a wet card after a short storage, and the natural conclusion is that the parts absorbed moisture, when the real problem was the packaging. Checking the desiccant unit count against the bag size, and using a fresh desiccant pack whenever a bag is re-sealed, keeps the card meaningful.

How the Card Sets the Bake Decision

The card is one input to the exposure calculation. The others are the moisture sensitivity level of the part, the time the bag was open, the ambient humidity on the floor and the time since the part was packed. Where the card reads dry but the floor life has already been exceeded during an earlier operation, a bake is still required.

Where the card reads wet, the bake is required regardless of the floor life, and the bake profile has to match the package thickness and the supplier instruction rather than a workshop default. Baking thin plastic packages at a temperature intended for thick ones damages the parts, and baking at too low a temperature leaves the moisture in place. The card identifies that a bake is needed; the package data sheet decides how it is done.

Handling, Storage and Card Shelf Life

Cards have a shelf life of their own and should be stored in a sealed container. A box of cards left open on a shelf will be partly changed before it is ever put in a bag, and every reading taken from that box is worthless. Stock should be rotated, the container kept closed and the card batch number recorded so that a faulty batch can be traced.

The same applies to the workshop environment. Opening a bag in a room at seventy percent relative humidity and then sealing it again with the same card guarantees a wet reading next time. Where the ambient cannot be controlled, the work should be done quickly, the desiccant replaced and the exposure time recorded, so that the next reader can see what happened rather than guess.

Common Reading Errors

The most common error is reading the card through the bag. The colour seen through a translucent film is not the colour of the spot, and the difference between a dry and a partly changed dot is exactly the difference that the film obscures. The bag must be opened before the card is judged.

The second error is reading the card after the bag has been open for hours, by which time a dry room has begun to dry the card again. The third is failing to record the reading at all, so that the next shift finds an opened bag with no explanation. The fourth is treating a card with a single changed spot as a pass because the change is small, which is how moisture damage enters a build quietly.

Records and Traceability

The reading should be written on the traveller or the material record with the part number, the date, the time the bag was opened and the initials of the person who opened it. Where a batch is large or the reading is marginal, a photograph of the card next to the bag label removes any later argument about what was seen.

Those records connect directly to the floor life calculation. The floor life budget for a part is consumed by every exposure, and a card reading is the only evidence of whether the sealed periods in between were dry. The tracking described in the moisture sensitive device record, the controls in the component moisture procedure and the packaging checks in the packaging material routine all depend on the same small card being read honestly.

FAQ

What humidity level on the card forces a bake? Most specifications require the ten percent spot to be unchanged when the bag is opened. If that spot has started to change, the parts are treated as having absorbed moisture and a bake is required before soldering, whatever the floor life calculation says.

Can a humidity indicator card be reused? No. Once the card has been exposed it is a record of that exposure and nothing more. A fresh card belongs in every bag that is re-sealed, together with fresh desiccant, or the next reading describes the workshop rather than the parts.

Why does the card read wet when the parts were never opened? The usual causes are a weak heat seal, a puncture in the bag, insufficient desiccant for the bag volume, or a card that was already partly changed before it was packed. All four are packaging faults, and all four are worth investigating before the parts are baked.