Deposit Volume: Solder Paste Deposit Measurement by Weight and Inspection

The quantity that decides a solder joint is the volume of paste that was deposited, and the volume is not directly visible. The two ways to measure it are to weigh it and to look at it.

Weighing the Deposit

The gravimetric method weighs the board before and after printing and divides by the number of apertures. It gives an average volume and it is quick.

An average hides the distribution, so it cannot show that one aperture is filling and another is not. It is useful as a process check rather than as a diagnosis. Our paste notes describe the density that converts mass into volume.

Optical Inspection

A paste inspection system measures the height of each deposit and computes the volume from the area. It gives a per aperture result and it is fast enough to run in line.

The measurement depends on the optical properties of the paste, so a calibration is required and a change of paste means a re-calibration. Our paste inspection notes describe the parameters that are set.

What the Numbers Mean

The volume should be compared with the requirement calculated from the joint, not with a nominal figure. The requirement depends on the pad, the lead and the fillet that the joint needs.

A deposit that is inside the control limits and outside the requirement is a controlled process producing the wrong joint. Our land pattern notes describe how the requirement is derived.

Setting the Limits

The limits should come from the distribution of a process that produces acceptable joints, which means they should be measured rather than copied from a supplier’s recommendation.

The lower limit is usually the more important one, because a short deposit produces an open or a weak fillet while a tall one produces a bridge that is visible in inspection. Our joint criteria notes describe the joint that results.

Correlation With the Joint

The measurement is a proxy for the joint, and the correlation should be established once by cross sectioning the joints that came from deposits at the limits.

Without that step, the limits describe the paste and not the joint. Our area ratio notes describe the aperture that sets the release.

Using the Result

Where the volume drifts through a run, the cause is usually the working life of the paste, the stencil condition or the printer. The three should be distinguished by measuring each.

Where the volume is short on the smallest apertures and correct elsewhere, the stencil is the answer rather than more pressure. Our stencil notes describe the condition that matters.

Records

The measurement should be recorded with the print parameters, the paste batch and the stencil identity, so that a change can be attributed rather than guessed.

Where the product is transferred between lines, the deposit measurement is the check that the transfer preserved the process. Our fabrication notes notes list the records that should be kept.

Process Control and Verification

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Where the requirement is not written down, the shop supplies its own default, and the default is chosen for the process rather than for the design.

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 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.

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 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.

Verification and Records

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. 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.

FAQ

Is weighing enough? It confirms the total and it hides the distribution. It is a process check rather than a diagnosis.

Does paste inspection need a new calibration for every paste? The optical response changes with the paste, so a re-calibration is required and the result should be confirmed against a weighed sample.

What does gopcb provide for deposit measurement? We provide a requirement calculated from the joint rather than a nominal, limits set from a producing process, correlation established by cross section, per aperture measurement in line, and records that tie the result to the paste, the stencil and the parameters.