Component Bake Schedules and Floor Life

A component that has absorbed moisture and is heated quickly can crack internally, and the crack is invisible from outside. The bake is the operation that removes the moisture before the heat arrives, and its schedule is set by the package rather than by the calendar.

Where the Moisture Sits

The mould compound and the die attach absorb water from the air, and the amount depends on the package thickness, the material and the exposure. A thin package equilibrates in hours and a thick one takes days.

That is why a large body part is more sensitive than a small one at the same sensitivity level. Our board quality notes describe how the resulting damage appears.

The Schedule and What Sets It

The schedule is a temperature and a time, and both come from the package data rather than from a general rule. A temperature that is too high damages the terminations and one that is too low takes an impractical time.

The schedule is specified for the package and the carrier, because a part on tape cannot be baked above the tape temperature. Where the tape limits the bake, the part is removed or the floor life is managed instead. Our profile notes describe the heat that follows.

Floor Life and the Clock

Floor life is the time a part can be out of the bag before it needs a bake, and it is set for the sensitivity level and the ambient conditions of the area. A part at the end of its floor life is not defective, it is due for a bake.

Tracking that time is the practical difficulty, because the clock starts when the bag is opened and it runs whenever the part is out of a dry store. Our first pass yield notes describe how a moisture related failure appears in the data.

A part can be baked a limited number of times before the terminations are affected. The count belongs in the record, because a part that has been baked repeatedly is a different part from one that has not.

The bake also has to be completed before the part is returned to the bag, otherwise the moisture returns and the operation is wasted. The rule is a bake, a return to the dry store or a bag, and a new clock.

The tracking is a label, a log or a system, and the important property is that it is visible at the station. A record that is kept in an office is a record that the line does not use.

The label carries the sensitivity level, the date of bag opening and the floor life that applies. A reel that carries a level and no date is a reel whose remaining life is unknown.

A bake is cheaper than a discarded reel and it is not free, because it consumes machine time and a bake cycle from the part’s allowance. The decision is made from the sensitivity level and the exposure rather than from the part cost.

A high sensitivity part at the end of its floor life is baked. A low sensitivity part in a controlled area is used as it stands, and the two decisions are made from the level rather than from a preference. Our paste volume notes describe the paste side of the same storage discipline.

Acceptance and Its Evidence

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.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.

The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.

A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first.

Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

Does a bake restore the part to new? It removes the moisture and it does not reverse a thermal excursion that has already happened.

Can a taped reel be baked? It can at a temperature the tape and the carrier tolerate, which is usually lower than the package allows.

What does gopcba provide for moisture control? We provide bake schedules set from the package data and the carrier limits, floor life tracked from bag opening with a label at the station, bake counts recorded, parts returned to a dry store after baking, and the decision to bake or use made from the sensitivity level.

Verification and Records

The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.