Wire Bonding and SMT Hybrid Assembly

A hybrid assembly carries wire bonded die and surface mount components on the same board. The two processes ask different things of the surface, and the sequence in which they are run decides whether both succeed.

What Wire Bonding Requires

A bond forms when heat, force and ultrasonic energy bring two clean metals into contact and let them interdiffuse. The requirement is therefore a surface that is clean at the atomic scale and a finish that bonds reliably.

Gold and aluminium are the common pairs, and each behaves differently at temperature. Our component selection notes describe the choices.

Bond Pad Finish and Intermetallic Growth

Gold over nickel is the usual finish for a gold wire bond, because the nickel limits the intermetallic growth between the gold and the copper beneath it. Without that barrier the bond degrades at temperature.

The growth rate rises steeply with temperature. Our thermal cycling notes describe the test that exposes it.

Cleaning and Surface State

A bond pad that has been handled, fluxed or stored in a humid room will not bond reliably. The cleaning step and the interval between cleaning and bonding both matter.

The same surface has to survive the reflow of the SMT side. Our cleanliness notes describe the measurement.

Thermal Budget and Sequence

Reflow comes first where the die can tolerate it, because the bonding that follows is a local, low temperature process. Where the die cannot tolerate the reflow, the sequence has to be reversed and the SMT profile becomes the constraint.

Either way the sequence should be written down. Our reflow notes describe the thermal mass problem.

Contamination From the SMT Step

Flux residue and outgassed material from the SMT step can deposit on a bond pad that is already finished. Where the pad is masked during reflow, the mask has to be removed without leaving adhesive.

That is one of the reasons the sequence is decided early. Our flux residue notes describe the risk.

Inspection and Test

A wire bond is inspected by pull test and by visual check for the bond shape, and the results are recorded per wire rather than per board. The record is what makes a later failure attributable.

X-ray is of limited use on a wire bond. Our inspection notes describe where each technique applies.

Verification

The verification is a bond pull test at the start and the end of a run, a cleanliness measurement before bonding, and a temperature exposure that represents the worst case the assembly will see.

Our quality notes describe how the records are kept.

Process Control and Verification

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.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one. A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.

The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting.

FAQ

Can a hybrid assembly be reworked? A bonded die is usually not reworkable once the intermetallic layer has formed. The decision is made at design rather than at repair.

Is ENIG suitable for bonding? It can be, where the gold thickness and the nickel beneath are controlled. A thin immersion gold over a well controlled nickel is the usual specification.

What does gopcb provide for hybrid assembly? We provide a stated bond pad finish with the barrier layer defined, a cleaning step with a measured interval before bonding, a process sequence chosen against the die thermal limit, a pull test recorded per wire, and a temperature exposure that represents the worst case in service.