Placement Force and Paste Displacement Control

A placement machine has to press the component into the paste hard enough to make it stick and softly enough to leave the deposit in place. The window is narrow, and it changes with the nozzle, the paste and the pad size.

What the Placement Force Does

The force seats the termination in the paste so that the tack holds the part until reflow. It also displaces the paste, and the paste that is displaced does not come back.

The result should be a part sitting on the deposit with the paste spread slightly around the termination. Where the paste is pushed aside, the joint forms with less material than the stencil provided. Our paste volume notes describe the deposit that the stencil controls.

How the Force Is Applied

The machine applies force through the nozzle, and the force depends on the programmed placement height, the stopping method and the nozzle springing.

A machine that places to a fixed height above the board applies a force that varies with the board thickness and the warpage. A machine that places to a force gives a more consistent result on a board that is not flat.

Paste Response

A stiff paste resists displacement and holds its shape after the part is placed. A soft paste flows more easily and can be pushed away from the pad by a heavy part.

The paste temperature and its age both change the response, so a paste that behaved correctly in the morning may behave differently at the end of a shift. Our paste notes describe the working life.

Part Mass and Pad Area

A heavy part on a small pad is the difficult case: the mass presses the paste out and the pad has little area to hold what remains.

The answer is usually more deposit rather than more force, and the pad and aperture design should account for the mass before the force is increased to compensate. Our land pattern notes describe how the pad is sized.

Bridging and Tombstoning

Excess force squeezes paste beyond the pad edge on a fine pitch part, and the displaced paste becomes a bridge after reflow.

Uneven force tilts the part, and a tilt on a two terminal chip component is the beginning of a tombstone. The placement height and the nozzle should be checked before the profile is suspected. Our defect notes describe the appearance of each fault.

Verification

The verification is the paste displacement around the placed part, measured on a sample before reflow, and the bridging and tombstone rate after.

Where the displacement is too great, the placement height should be raised before the stencil is changed, because the stencil change affects every joint on the board. Our inspection notes describe how the result is classified.

Machine Setup and Records

The placement height and the force should be recorded per product and per nozzle, because a change of nozzle changes the effective force.

Where a product is transferred between machines, the settings should be transferred with it and verified by the same displacement check rather than assumed to be equivalent. Our fabrication notes notes list the setup 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.

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.

Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected.

FAQ

Is more force always better for seating? No. Beyond the point where the part is seated, additional force removes paste and creates defects that did not exist in the deposit.

Why does the same setting work on one machine and not another? The nozzle length, the head springing and the board support all differ, so the force that reaches the part is not the same even when the display shows the same value.

What does gopcb provide for placement force control? We provide placement height and force development per product and nozzle, force based placement where the board flatness varies, paste response review with the working life, mass and pad area assessment, paste displacement measurement before reflow, and setup records that transfer with the product.