Wire Bonding Versus Solder Assembly for Modules
A module can be built by soldering components to a board, by bonding a die to a substrate and wiring it, or by a combination of the two. The two routes have different cost structures, different reliability behaviours and different minimum volumes, and the choice is usually made on the first two rather than on the third.
What Wire Bonding Buys
A wire bond connects a die pad to a substrate pad with a fine wire, and the connection is made without solder and without the flux that solder requires. That removes a cleaning step and a residue from the interconnection, which matters for a cavity that will be sealed.
It also allows a die to be placed where a package would not fit, and it tolerates a die that is available only in wafer form. Our thermal cycling notes describe how the connection is qualified over life.
What Solder Buys
Solder assembly uses the same process as the rest of the board, which means the same equipment, the same inspection and the same rework. That is the reason a module that can be built with solder usually is.
The joint is also self-aligning and inspectable, and its failure modes are known and modelled. Our joint criteria notes define the acceptance that the process is held to.
Die Attach and Its Material
Before a die is wired or soldered, it is attached to the substrate, and the attach material decides the mechanical and thermal path. A solder attach gives a low thermal resistance and a stiff connection; an epoxy attach is more compliant and easier to process.
The attach also sets the void content under the die, which is the item that decides the junction temperature. That is why the attach is qualified by thermal measurement rather than by a pull test alone.
Reliability of the Two Routes
A wire bond fails by fatigue at the heel of the bond or by intermetallic growth at the interface, and both are accelerated by temperature cycling. A solder joint fails by fatigue in the bulk or at the intermetallic, and it is accelerated by the same cycles.
The difference is in the mechanism and in the temperature at which it becomes significant. A gold aluminium bond grows intermetallics at a temperature that a solder joint would tolerate, which is why the wire bonded module has a lower maximum temperature unless the materials are chosen deliberately.
Inspection and Rework
A solder joint is inspected optically and by X-ray, and a wire bond is inspected by pull and shear tests on a sample plus a visual check of the bond shape. The wire bond test is destructive, so it is applied to a coupon rather than to the product.
Rework differs even more. A solder joint can be reflowed; a wire bond is removed and re-made, and the pad may only tolerate a limited number of reworks. That limit belongs in the process instruction.
Sealing and the Cavity
A wire bonded module is often placed in a sealed cavity, and the sealing method has to be compatible with the bonds. A sealed cavity also changes the failure modes, because moisture that is trapped inside cannot leave.
A solder assembled module is usually conformally coated rather than sealed, and the coating has to reach the joints. Our coating notes describe how that coverage is verified.
Choosing Between Them
The choice is made on three questions: whether the die is available in a package, whether the environment exceeds what a solder joint will take, and whether the volume justifies the tooling that wire bonding needs.
Where all three point the same way the decision is easy, and where they conflict the answer is usually a hybrid: wire bonds inside a sealed cavity and solder everywhere else. Our board quality notes describe how the two are judged together on one assembly.
Process Control and Verification
On a design of this kind, die attach is the item that decides how the rest of the board is arranged. 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.
Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Process Control and Verification
On a design of this kind, die attach is the item that decides how the rest of the board is arranged. 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.
Process Control and Verification
On a design of this kind, die attach is the item that decides how the rest of the board is arranged. 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.
FAQ
Is a wire bond stronger than a solder joint? It is smaller and it carries less current per connection, so the comparison depends on what the connection has to do.
Can a wire bonded module be reworked? Yes, within a limit that is set by the pad and recorded in the process instruction.
What does gopcb provide for module assembly? We provide die attach selection with the void content measured thermally, wire bonding with pull and shear data from coupons, solder assembly with the same inspection standard as the rest of the board, sealing or coating matched to the application, and qualification by thermal cycling.
