Clean or No-Clean: Making the Residue Decision
The choice between cleaning and leaving the flux residue is made on cost, and it should be made on the requirement. The two are not equivalent processes and the difference appears later.
What No-Clean Means
A no-clean flux is formulated so that the residue it leaves is not harmful in the conditions the product is expected to see. That is a conditional statement, not an absolute one.
The residue is designed for a defined application amount and a defined profile. Applied too heavily or processed outside the profile, the residue is no longer what was qualified. Our flux notes describe the amount that is controlled.
Where No-Clean Is Not Enough
Where the product sees humidity and a bias voltage, the residue is one of the inputs to the electrochemical processes that cause leakage. Our migration notes describe the mechanism.
Where the assembly will be coated, the coating adheres to the surface rather than to the residue, and the residue can interfere with the bond. Our <a href="https://www.gopcba.com/coating-adhesion-surface-prep/” title=”adhesion”>adhesion notes describe the requirement.
What Cleaning Costs
Cleaning costs the equipment, the chemistry, the water and the energy, and it adds a step where the fluid has to reach every surface and then be removed.
It also introduces a risk: a rinse that is not complete leaves the cleaning chemistry behind, which is often more harmful than the flux. Our cleanliness notes describe the measurement that detects it.
The Decision Inputs
The inputs are the residue chemistry, the amount applied, the applied voltage, the expected humidity, whether the assembly will be coated, and the requirement of the customer or the standard.
Where the requirement is not stated by a standard, it should be decided by analysis rather than by default. The analysis should be recorded as a decision with its inputs.
Cleaning Technologies
An aqueous process removes ionic residue well and it needs a rinse and a dry that leave no water behind. A solvent process suits the fluxes that water does not dissolve.
A semi-aqueous process uses a solvent to dissolve and water to rinse, which adds a step. Each technology has a residue it handles well and a geometry it cannot reach. Our moisture notes describe the exposure the cleanliness is managed for.
Where the Fluid Cannot Reach
Under a low clearance package, inside a connector and in a blind area, the fluid may not reach the residue and the residue may not be rinsed out.
Where the geometry prevents cleaning, the process should prevent the contamination instead by controlling the flux and the amount applied.
Verification
The verification is the cleanliness measurement on production output, and the decision should be reviewed when the flux, the product or the requirement changes.
Where the process is no-clean, the verification is the process control that keeps the residue within what was qualified, rather than a cleanliness measurement that would fail by design. Our sampling notes describe how the control is checked.
Additional Considerations for This Build
Practical attention to ionic contamination pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating ionic contamination explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, cleaning 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. 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.
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.
Process Control and Verification
On a design of this kind, cleaning 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, cleaning 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.
Process Control and Verification
On a design of this kind, cleaning 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.
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
Is a no-clean process cheaper? It avoids the cleaning equipment and it requires tighter control of the flux, the amount and the profile. The saving is in capital rather than in control.
Can a no-clean assembly be coated? It can where the residue is compatible with the coating, which should be verified by an adhesion test rather than assumed.
What does gopcb provide for the clean decision? We provide a decision based on residue chemistry, application amount, bias and humidity, coating compatibility and the applicable requirement, a recorded decision with its inputs, cleaning processes with rinse and dry control, and cleanliness measurement on production output.
