Exposed Pad Soldering: 6 Rules for a Flat Joint

An exposed pad is the metal land on the underside of a package that carries heat out of the die and into the board. It is soldered at the same time as the pins around it, yet it behaves like a completely different joint, because it is large, it is hidden from view and it has to be void free to move heat properly.

The failure mode is quiet. A part with a partly soldered pad still passes an electrical test, because the signal pins are connected and the die works at bench power. The problem appears later as a hot component, a drifting parameter or a joint that cracks after a few hundred thermal cycles.

Why an Exposed Pad Is Hard to Solder

The land is large compared with the metal around it, so it takes longer to heat and it cools at a different rate from the pins. The paste on the pad also has a long way to travel before it coalesces, and any flux gas that forms underneath has nowhere to escape once the part settles onto the alloy.

Weight works against the joint as well. A power package is heavier than a fine pitch device, and its own mass presses the molten alloy across the land while the solder is still liquid, which moves the voids around rather than removing them.

Stencil Opening and Paste Volume

The solder paste volume on the land is set by the stencil opening, and the usual target is a deposit that fills the joint without squeezing out at the edges. Too little paste leaves a partial joint, while too much pushes alloy onto neighbouring pads during placement.

Area ratio still governs how cleanly the paste leaves the opening, and a large opening is usually easy in that respect. The constraint on a large land is not release but volume control, which is why the design is often split into a pattern rather than left as one rectangle. Our paste release notes cover the geometry behind the opening.

Splitting the Paste Into a Grid

A land printed as a grid of small openings gives the gas a route out between the deposits and produces a more even fillet after reflow. The pattern is drawn on the stencil, so it costs nothing at assembly and it is one of the cheapest improvements available to a difficult joint.

The grid should be sized so that the individual deposits coalesce during reflow. Openings that are too far apart leave separated islands of alloy, which is a worse result than a single large deposit, so the spacing should be checked against the recommended pattern for the package.

Placement Pressure and the Part

A part placed with too much force pushes paste out from under the land before reflow begins, and the loss is invisible because the pad is hidden. The placement force should be set for the package, and the nozzle should be wide enough to sit on the body rather than on the leads.

The placement height matters as well, because a part that is pressed too deep bottoms out on the paste and lifts slightly as the head withdraws. A first article with a removed part, placed and then taken off before reflow, shows exactly how much paste remains under the pad.

Void: What Causes It and What It Costs

A void is a pocket of gas or a region where the alloy never wetted the land. It reduces the area that carries heat, and the reduction is not linear: a joint with a quarter of its area missing can run several degrees hotter than a sound one, which is enough to derate the part.

The usual sources are flux gas trapped under the part, an oxide layer on the land, and paste that has dried before placement. All three are process variables rather than material faults, and our void causes notes list the checks in the order they should be made.

Reflow Profile for a Large Land

A large land needs a longer soak than a small pad, because the thermal mass of the copper under it holds the temperature down. A profile developed on a test coupon with a small land will run cold on a real board, and the joint that results is the classic partial wetting case.

The profile should be verified with a thermocouple under the part if the board allows it, or with a thermal simulation supported by a measurement at the land edge. Time above liquidus should be long enough for the alloy to spread across the land rather than simply change state.

Checking the Joint Without Breaking It

An X-ray view is the practical check, because the joint cannot be seen from outside. The image shows the void pattern, the extent of wetting and any paste that squeezed beyond the land, and comparing a production board with a known good reference makes the judgement repeatable.

The inspection program should be set with a void limit that the design can tolerate, rather than a limit copied from another product. Our QFN inspection notes describe how the image is read and what the different patterns mean.

Rework and Removal of a Bonded Part

Removing a part with a bonded thermal land takes more heat than removing an ordinary package, and the board around it is at risk. Preheating from below, a controlled hot gas cycle and a support under the board all reduce the chance of delamination or pad lift during the operation.

The land must be cleaned back to bare metal before a replacement part is fitted, because a mix of old and new alloy raises the melting range and produces a joint with a different void pattern. The cleaning method should be gentle enough to leave the mask and the neighbouring pads intact.

Design Choices That Reduce Risk

Thermal vias under the land carry heat into the board and give the gas a path away from the joint, and they are most effective when they are tented on the opposite side or plugged rather than left open. Their number and spacing are set by the thermal requirement, not by the paste pattern.

Thermal pad size should follow the package recommendation rather than the largest area the space allows, because an oversized land increases the volume of alloy that has to be heated and gives the gas more room to be trapped. The routing around the land, and the copper balance on the opposite side, belong in the same review.

Records and Process Control

The record for this joint should hold the stencil pattern, the paste volume, the profile and the void result from the first article. Those four items, kept together, allow a change in defect rate to be traced to a change in the process rather than debated as a supplier problem.

Every new package should be treated as a new process rather than an extension of an existing one, and our first article notes describe how that check is run. The IPC assembly standards give the acceptance criteria for the void limits used in the specification.

FAQ

Can the void be eliminated completely? In practice no. A small percentage of void is normal and acceptable, and the specification should state the limit the thermal design can tolerate. Chasing zero void usually leads to more paste and a worse result at the edges.

Is a nitrogen atmosphere necessary? It helps on a large land, because it slows oxide formation while the alloy is spreading. Where a board has a difficult land and a thin paste deposit, the improvement is usually visible in the void pattern before it is visible anywhere else.

How is a partly soldered pad found in production? Through X-ray on a sample, and through the thermal test on the finished assembly. A functional test at room temperature will not separate a good joint from a poor one, which is why the sample check carries the weight.