Bottom Terminated Components: Placement and Reflow Guide

Bottom terminated components cover a wide family: quad flat no lead packages, dual flat no lead parts, land grid arrays and the many power packages whose only connection is a pad underneath the body. They are attractive because they are small and because the thermal pad beneath the die gives a short path for heat, and they are difficult because every joint is hidden and the thermal pad has to be soldered without leaving voids.

What Counts as a Bottom Terminated Component

The family is defined by the terminations rather than by the package outline. A quad flat no lead part has lands around the perimeter and usually an exposed thermal pad in the middle, a dual flat no lead part has lands on two sides, and a land grid array has an area array of pads with no balls.

What they share is that the solder joints are formed under the body and cannot be seen from above. Inspection depends on X-ray or on a careful design of the land pattern, and the process window is narrower than that of a gull wing part because there is no lead to absorb a difference in height.

Pad and Stencil Design for the Thermal Pad

The land pattern should follow the package drawing, with the perimeter pads sized to give a visible fillet after reflow, since that fillet is the only external evidence that the joint formed. The thermal pad is normally divided into a grid of smaller openings rather than one large aperture.

Dividing the paste into a grid does two things: it reduces the volume of paste under a large area, and it leaves channels through which the volatiles can escape during reflow. A single large aperture produces a heavy deposit, a large volume of gas and a correspondingly high void content.

Paste Volume and Aperture Strategy

The paste volume for the perimeter joints is set by the aperture area and the stencil thickness in the usual way, but the thermal pad is a special case. A deposit that is too thin leaves an incomplete joint, while one that is too thick lifts the part and opens the perimeter joints.

The area ratio of each small aperture governs its release, so the grid has to be designed with apertures that release well rather than with the largest openings that will fit. A common approach keeps the total paste area for the thermal pad between roughly fifty and seventy percent of the pad area.

Placement Force and Coplanarity

The component has to sit flat on the paste, and the placement force is what pushes it down. Too little force leaves the part riding on top of the deposit, while too much squeezes paste out sideways and can bridge the perimeter lands.

Coplanarity is the other half of the problem. A package whose thermal pad sits a few micrometres lower than the perimeter lands will not touch the paste in the middle, and one whose pad protrudes will lift the lands clear of their paste. The tolerance comes from the package supplier and should be checked against the placement force the machine uses.

Reflow Profile Considerations

The profile for a bottom terminated part needs a soak long enough to drive off the volatiles from the large paste deposit and a peak long enough to melt the alloy under the body. A profile that works well for fine pitch gull wing parts may be too fast for the thermal pad.

The mass of the package matters as well, because a power device with a metal slug takes longer to reach temperature than its size suggests. Measuring the profile with a thermocouple under a dummy part is the usual way to confirm the setting before production begins.

Voiding Under the Thermal Pad

Voids form under the thermal pad for the same reasons they form anywhere: volatiles trapped by a large deposit, poor wetting, or a pad that has oxidised. Their effect is greater here because the pad is part of the thermal path and a void reduces the area available for heat to flow through.

Acceptance criteria should come from the thermal requirement, with a limit on the total void area and often a separate limit on the largest single void. Vacuum reflow is sometimes used to reduce the content further, and the same measurements are used to judge whether it has helped.

Inspection and X-ray Criteria

X-ray is the standard inspection method, and the useful views are a top down image for the perimeter joints and the thermal pad, plus an angled view of the heel of the joint where a crack or an open is most likely. Optical inspection can check the fillet and the placement but cannot see under the body, as the guide to automated optical inspection explains.

The images should be compared against a known good board rather than judged in isolation, because the appearance of a joint varies with the alloy, the finish and the X-ray settings. The comparison of the two methods is set out in the guide to X-ray and AOI inspection.

Rework of Bottom Terminated Parts

Rework follows the same sequence as any area array package: preheat from below, hot air or infrared from above, removal when the alloy is molten, and dressing of the site. The thermal pad makes removal harder because the solder volume is larger and the pad holds heat away from the perimeter.

Replacement needs a stencil or a preform to deposit the right volume of paste on the thermal pad, since hand application rarely gives an even layer. The joint quality after rework should be verified by X-ray, and the site should be checked for lifted pads before the new part is placed.

Design and Process Checklist

A rejected board is the most useful document in the review, so the checklist should cover the land pattern and its relationship to the package drawing, the stencil apertures for the thermal pad, the paste volume, the placement force, the profile and the void criterion. It should also record the package supplier and the coplanarity figure, since a change of supplier can change the assembly result.

On the process side, first articles should be X-rayed or sectioned before the build continues, and the results kept with the setup record. Where a thermal pad has to carry a defined power, the requirement that drives the void limit is described in the guide to thermal management design.

FAQ

Why is the thermal pad paste divided into squares? To reduce the total paste volume and to give the volatiles a path out during reflow. The divisions also make it easier to keep the deposit even across a large area.

Can these parts be inspected optically? The perimeter fillet and the placement can be checked optically, but the joints under the body cannot be seen. X-ray is needed for the thermal pad and for any judgment about voids or opens.

How flat must the package be? Flat enough that the thermal pad and the perimeter lands both make contact with their paste within the placement force available. The supplier figure should be compared with the machine setting before the part is designed in.