Mixed Alloy Soldering and Step Reflow Windows

Two soldering alloys in the same assembly is a common requirement: a lead free main assembly with a temperature sensitive part, or a mixed alloy legacy board. The problem is that the second alloy must melt without disturbing the first.

Why Mixed Alloys Appear

A part that cannot take the lead free reflow temperature, a board with a low glass transition, or a legacy assembly that was qualified with tin lead all create a mixed alloy situation.

The intention is a step soldering process where the lowest melting alloy is used last so that each earlier joint remains solid. The intention is not always what happens, because the alloys mix. Our paste notes describe the handling that keeps the two separate.

Melting Behaviour

A eutectic alloy melts at one temperature. A non eutectic alloy melts over a range, and partial melting begins at the solidus rather than at the peak. The difference matters because a joint can soften long before it flows.

When two alloys meet, the joint composition becomes something between them, and the melting range of the mixture is not the melting range of either. The result is a joint that can move at a temperature the process was not expecting.

Low Temperature Alloys

A bismuth containing alloy melts well below the lead free peak, which is what makes it attractive for temperature sensitive assemblies. It is also brittle compared with tin silver copper, and it is sensitive to the presence of lead.

A small amount of lead contamination raises the melting range of a bismuth alloy sharply, so the two must not be mixed by rework or by a contaminated tool. Our joint defects notes describe the failures that follow.

Process Windows

The profile must melt the second alloy completely while keeping the first below its solidus. The window is the difference between the two, and it should be measured on the assembly rather than calculated from the alloy chart.

Where the window is narrow, the board and the components must be characterised, because the actual temperature at the joint lags the oven setting by an amount that depends on the thermal mass.

Rework in a Mixed Assembly

Rework is where the two alloys usually meet. A tool used on a lead free joint and then on a bismuth joint carries the first alloy into the second, and the melting range rises.

The tools should be dedicated, or cleaned and verified between the two, and the rework temperature should be set for the lower alloy. The result should be inspected for a change in the joint appearance rather than only for continuity.

Reliability Considerations

A mixed joint has a different fatigue behaviour from either of the alloys. Where the assembly will see thermal cycling, the mixed joints should be tested as a population rather than assumed to behave like the parent alloy.

The test should include the joints that were reworked, because those are the ones whose composition changed. Our reliability notes describe how the sample is defined.

Documentation

The drawing should state the alloy for each process step, the peak temperature limit for the parts that constrain it, and the order of the operations.

Where the assembly will be repaired in the field, the repair instruction must state the same limits, or the first field repair will exceed the temperature the sensitive part can take. Our fabrication notes notes list the attributes that should be stated.

Process Control and Verification

On a design of this kind, solidus is the item that decides how the rest of the board is arranged. 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.

Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch.

Process Control and Verification

On a design of this kind, solidus is the item that decides how the rest of the board is arranged. 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.

Process Control and Verification

On a design of this kind, solidus is the item that decides how the rest of the board is arranged. 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, solidus is the item that decides how the rest of the board is arranged. 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 first article check confirms that the process and the drawing agree on the points listed above, and that the coupon data supports the values used in the design.

Where a measurement falls outside the expected window, the sample is retained so that the cause can be established before the balance of the batch is released.

FAQ

Can a lead free board be reworked with tin lead solder? It can be done, and it produces a joint that must be treated as a different material with a different melting range. The mixture should be tested rather than assumed.

Is a low temperature alloy always the safe choice? It is the safe choice for the temperature sensitive part and it introduces a new set of mechanical limits. It is a trade rather than a solution.

What does gopcb provide for mixed alloy assemblies? We provide alloy selection and step order, measured process windows on the real assembly, dedicated tooling and cleaning rules for rework, joint appearance criteria for the mixed composition, thermal cycling test of the reworked population, and documented limits for both production and field repair.