Parallel Seam Welding Quality for Sealed Packages

A sealed electronic package has to stay sealed. Where the lid is metal and the body is metal, the joint is made by welding, and the process used for most high reliability packages is parallel seam welding. Two electrodes roll along opposite edges of the lid while a current passes between them, heating the interface and forming a continuous weld that is both a mechanical joint and a hermetic seal.

The process is fast, produces a weld with no filler material and can be automated for high volume production. It is also sensitive to a long list of variables, and a seal that passes a fine leak test immediately after welding can fail after thermal cycling if the weld nugget is not fully formed. Understanding what controls the weld is the basis of controlling the seal.

How the Weld Is Formed

The current path runs from one electrode, through the lid, across the lid to body interface and back through the second electrode. The resistance at the interface is the highest in that path, so most of the heat is generated there. The electrodes are also cooled, which keeps the lid surface from overheating while the interface reaches welding temperature.

Because the electrodes are on opposite sides of the package and move together, the weld is made in two places at once, one on each edge. The result is a seam whose width depends on the electrode tip geometry, the applied force and the current. The nugget has to penetrate both the lid and the body without melting through the lid, which sets a window for all three parameters and makes the process sensitive to the thickness of the parts.

Parameters and Their Interactions

The controllable parameters are current, pulse duration, electrode force, roll speed and electrode geometry. They interact: a higher current with a faster roll speed can produce the same energy per unit length as a lower current with a slower speed, but the two do not produce the same weld, because the time available for heat to diffuse into the material is different. A fast, high current weld produces a narrower nugget with a sharper thermal gradient than a slow, low current one.

Electrode force controls the contact resistance and therefore the heat generation, and it also determines how much the parts are deformed. Too little force and the contact is unstable, producing spitting and an inconsistent nugget; too much and the lid is crushed, which can damage the package or the die inside. The force is applied by a pneumatic or a servo system, and its stability over a production shift is as important as its set point.

Electrode Condition and Wear

The electrodes are consumable. Their tips flatten and pick up material from the lid, which changes both the contact area and the current density. A worn electrode produces a wider weld than a new one and, because the current density falls, a shallower nugget. Because the change is gradual, it can pass inspection for a long time and then produce a batch of packages with marginal seals.

Managing the wear is a matter of counting the welds and dressing or replacing the electrodes at a defined interval, rather than waiting for a visible defect. The dressing schedule should be established by a weld quality test, such as a peel or a cross section on a sample, at several points in the electrode life. Where the package material changes, the wear rate changes with it, and the interval should be re-established rather than carried over.

Materials and Plating

The weldability of the lid and body depends on their material and on the plating. Kovar, alloy 42 and stainless steel are common lid materials, and each has a different resistivity and thermal conductivity, which shifts the process window. Gold plating, used to protect the surface, dissolves into the weld and can cause embrittlement if the plating is thick, so the thickness is specified as a maximum as well as a minimum.

Cleanliness matters at the interface. An oxide layer or an organic film raises the contact resistance and makes the weld unstable, and contamination trapped in the weld becomes a leak path later. Cleaning before welding, and controlling the interval between cleaning and welding, are part of the process rather than preparations for it. The atmosphere in the welding chamber is also controlled, both to protect the weld and to keep the package interior dry, which is the same concern that drives moisture sensitivity handling on conventional assemblies.

Inspection and Hermeticity Testing

Weld quality is assessed in two ways. The first is a destructive test on samples: a peel test to measure the width and the penetration of the nugget, or a cross section to measure its depth and to look for porosity. The second is a non destructive hermeticity test on every unit. A fine leak test with a tracer gas detects a through path, while a gross leak test detects a larger opening, and the two are complementary because neither detects the full range of leak sizes.

The leak rate specification comes from the package standard and depends on the internal volume, and it is usually tighter for a small package than for a large one. Passing a leak test at the time of manufacture is not the end of the evidence, because a marginally sealed package may pass and then open during temperature cycling. A qualification programme that combines leak testing after thermal cycling and after mechanical shock is what demonstrates that the weld, and not only the inspection, is adequate.

Process Control in Production

Because the process has many interacting variables, control is maintained by monitoring rather than by setting. Current and voltage waveforms for each weld can be recorded and compared against a reference, and a deviation indicates a change in contact resistance, in the parts or in the electrodes. This is a more sensitive indicator than a visual inspection of the finished seam, and it is available on every unit rather than on a sample.

The records should also include the electrode count, the force verification and the results of the periodic destructive tests, so that a change in leak rate can be traced to a specific cause. Assembling that evidence over time is what turns a welding operation into a controlled process, and it is the same principle that governs any other production step where the result cannot be inspected directly, as in the wider set of pcb manufacturing processes.

Fixtures and Part Location

The weld follows the path of the electrodes, so the electrodes have to follow the seam. The fixture locates the package so that the lid edge is directly under the electrode path, and any positional error shows up as a weld that is offset towards the lid or towards the body. Because the nugget has to penetrate both parts, an offset reduces the penetration on one side and produces a joint that looks continuous but is weaker than intended.

The fixture also has to hold the lid in contact with the body. A lid that is domed, or a body whose sealing surface is not flat, leaves a gap that the electrode force has to close before welding can begin. If the force is used to close that gap, the metal is deformed rather than welded, and the resulting seam may pass a leak test because it is mechanically tight while having no metallurgical bond at all. Measuring the flatness of the sealing surfaces on incoming parts is a cheap way to avoid that failure.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Why does a package pass a leak test and fail later? Because a marginally welded nugget can open under thermal cycling. The qualification test should include cycling followed by a leak test, not a leak test alone.

Can the weld be repaired? Usually not. Rewelding an already welded seam tends to produce a wider, weaker joint with more contamination, which is why the process is controlled to avoid the need.

What is the most common cause of a change in weld quality? Electrode wear, followed by a change in the plating thickness of the lid. Both shift the contact resistance and therefore the heat generated at the interface.