Assembly Area Humidity and Dew Point Control

An assembly area is controlled for humidity because the paste, the components and the coating all respond to it. The measurement that matters is taken where the work happens rather than at the air handling unit, and the two can differ by more than the control band.

Why the Assembly Area Is Controlled

The paste gains or loses solvent depending on the humidity, the components absorb moisture and the coating cures differently. Each of those has a window, and the window is set by the material rather than by comfort.

That is the reason the control is specified as a range rather than as a maximum. A very dry room dries the paste and a very humid one wets it. Our board quality notes describe how the resulting defects appear.

Measuring Where the Work Happens

A sensor at the air handling unit describes the supply and not the room. The measurement that matters is taken at the printer, at the placement machine and at the coating station, because the equipment generates heat and the local conditions differ.

Several sensors are better than one, and the record is the local figure rather than the average. Our paste volume notes describe the print that the measurement is supporting.

Dew Point and Condensation

The dew point is the temperature at which the air reaches saturation, and it is the figure that predicts condensation. A board that is brought into a warm room from a cold store condenses water if its surface is below the dew point.

That is why a cold board is conditioned before it is handled, and the conditioning period is set from the dew point rather than from a habit. The same applies to a component and to a paste jar.

Coating cures by losing solvent or by reacting, and both are affected by the ambient humidity. A coating applied in a humid area cures at a different rate from one applied in a dry area, and the difference appears as a tacky surface or as a blush.

The cure schedule is therefore set with the room condition as a variable. Our coating notes describe the application side of that.

The record is the local temperature and humidity at each station, taken continuously or at intervals, and reviewed as a trend rather than as a pass or fail. A room that oscillates around the limit is a room that produces a print that oscillates.

The trend is what explains a yield change that has no other cause. A yield that falls when the weather changes is a yield that is being controlled by the weather. Our cleanliness measurement notes describe a related measurement that is also worth trending.

The controls are a controlled room with local measurement, a conditioning period before a cold item is opened, a coating cure schedule that names the room condition, and a trend that is reviewed rather than filed.

Those four address the cause and they are cheap relative to the defects they prevent. A board that is scrapped for a humidity related print defect costs more than the sensor.

Acceptance and Its Evidence

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.

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. 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.

Checks Before Release

Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record. A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed.

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. 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.

Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

Verification and Records

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.

Is a drier room always better? No. A very dry room dries the paste at the stencil and changes the print.

Does the air handling reading describe the room? It describes the supply, and the local conditions at the equipment differ from it.

What does gopcba provide for area control? We provide a range set from the material requirements, measurement at the printer, the placement machine and the coating station, dew point used to set conditioning periods, the coating cure schedule written with the room condition, and a trend reviewed rather than filed.

Points to Confirm at First Article

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. 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.