QFN and Bottom Termination Component Soldering

A quad flat no lead package sits on its pads rather than on leads, so the joint is formed at the edge of the package and it is short. The joint that forms is often thinner than the design assumed.

Why the Joint Is Different

The package body rests on the paste as it melts, so the final standoff is set by the paste volume rather than by a lead. Too little paste and the joint is a sliver; too much and the package floats.

The wetting is also different, because the solder has to climb the side of the termination where there is little height to work with. Our paste volume notes describe the control of one of the two inputs.

Thermal Pad Voiding

The exposed pad in the centre carries the heat and it is the largest single joint on the package. Voids in it raise the thermal resistance, which raises the junction temperature.

The escape path for the flux volatiles is long in the centre of a large pad, which is why the aperture is usually split. Our <a href="https://www.gopcba.com/solder-void-vacuum-reflow/” title=”void”>void notes describe the measures.

Paste Volume and Aperture Design

The paste for the perimeter pads and the paste for the thermal pad are two different requirements on one stencil. The perimeter needs a controlled volume for a thin joint and the centre needs enough to fill the gap.

Where the two conflict, a step stencil or a different aperture ratio resolves it. Our area ratio notes describe the limit.

Placement and Self Alignment

The package aligns itself during reflow as long as the surface tension is symmetric. A paste volume imbalance between opposite sides pulls the package and it stays there.

The placement accuracy therefore has to be good and the paste has to be symmetric, which means the stencil aperture is the more important of the two. Our placement notes describe the measurement.

Inspection

The toe of the joint at the package edge is the only part that can be seen optically, and it should show a fillet that wets the termination and the pad.

The X-ray shows the thermal pad voids and a bridging defect under the package. Neither method confirms the interface at the pad. Our X-ray notes describe what can be resolved.

Rework

The package must be removed with heat applied from the bottom and the top together, because the thermal pad connects it to the board. Local heating from the top alone usually leaves the package attached by the centre.

The pad must be cleaned and the pasted again before replacement, and the same pad must survive the second rework. Our rework notes describe the methods.

Process Control and Verification

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.

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.

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. 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.

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.

Verification and Records

Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. A measurement taken at the wrong point of the process describes the wrong thing, however carefully it is made.

The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold. Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure.

Where a decision is made by judgement, a boundary sample makes the judgement repeatable between operators and between shifts. 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.

FAQ

Can the joint be judged from the toe fillet? The fillet confirms that solder is present and wetting. It does not confirm the thickness or the interface.

Is a large thermal pad void always a problem? It is a problem where the pad carries heat. The limit should come from the thermal requirement rather than from a general figure.

What does gopcb provide for QFN assembly? We provide aperture design that separates the perimeter and thermal pad requirements, split thermal apertures that shorten the escape path, symmetric paste for self alignment, bottom and top heating for rework, and X-ray and optical inspection matched to what each can resolve.