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

On a design of this kind, QFN is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, QFN is the item that decides how the rest of the board is arranged. 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. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel.

Process Control and Verification

On a design of this kind, QFN is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, QFN is the item that decides how the rest of the board is arranged. 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 first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

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.