Solder Mask for Microwave Assemblies and Soft Soldering
A microwave assembly is soldered with the same tools as any other board, but the requirements are different in one important respect: the solder mask is no longer only a protective coating, it is part of the electrical environment. At frequencies above a few gigahertz the mask sits in the field of the transmission line, and its thickness and dielectric properties affect the impedance, the loss and the behaviour of the structure around it.
There is a second reason to treat the mask carefully in these assemblies. Soft soldering of connectors, substrates and shielding cans involves fluxes that are difficult to remove, and the mask determines where the flux goes, what it can reach and how completely it can be cleaned afterwards. Getting the mask right is therefore a question of both electrical performance and process control.
Why Solder Mask Affects Microwave Performance
A microstrip line is a conductor over a ground plane with air above it. When the mask covers the conductor, the effective dielectric constant of the medium around the line rises and the impedance falls. On a fifty ohm line with a thin dielectric this can shift the impedance by a few ohms, which is tolerable in a narrow band application and can be significant in a broadband one. The effect grows with frequency and with mask thickness.
Dielectric loss is affected as well. Mask materials have a higher loss tangent than the laminate beneath them, and the field concentration near the conductor edge means that a thin layer can contribute a disproportionate share of the total loss. In a filter or a coupler where the geometry is tuned, the mask also shifts the resonant frequency, which is why prototypes are sometimes measured with and without the mask to establish the offset before production.
Mask Openings and Controlled Impedance
The standard remedy is to keep the mask off the critical transmission lines. An opening over a microstrip line removes the dielectric loading and leaves the impedance close to the value calculated for the bare laminate, at the cost of losing the protection against oxidation and against solder bridging. Where the line carries no exposed pads, the trade is usually worth making.
Where an opening is not possible, the mask thickness should be accounted for in the impedance calculation rather than ignored. The thickness of a screen printed or sprayed mask over a conductor is not uniform, and over a narrow trace it tends to be thinner than over a wide area. Specifying the mask as a controlled thickness, and asking the fabricator to confirm it, is more reliable than assuming a nominal value. The general treatment of impedance along a structure is the same as any impedance discontinuity analysis.
Mask and Soft Soldering
Soft soldering with a flux activated at a moderate temperature is used to attach connectors, substrates and shields, and the flux has to wet the metal surface but not the mask. A well cured mask resists the flux and acts as a dam that keeps it in the joint area, which makes cleaning easier and reduces the chance that flux reaches a region where it cannot be removed.
The word cured is important. A mask that is under cured softens in contact with the flux and with the alcohol used for cleaning, and it can lift at the edges of an opening. Lifted mask near a high frequency structure changes the geometry and, if it lifts over a line, changes the impedance. Specifying the cure schedule and confirming it with a solvent resistance test is a reasonable control for an assembly that will be cleaned aggressively.
Flux Residue and Cleaning
Flux residue is a dielectric, and in a microwave assembly it sits in the most sensitive part of the circuit. Residues around a connector launch or across a filter structure shift the response, and they also absorb moisture over time, which makes the shift unstable. Cleaning is therefore not cosmetic in these assemblies; it is part of the electrical specification.
The cleaning method has to be chosen with the substrate in mind. Ultrasonic cleaning is effective but can damage wire bonds and thin substrates, and spray cleaning may not reach under a shield can. The practical approach is to design the assembly so that the flux has nowhere to hide, using mask as a dam and avoiding enclosed volumes that cannot be flushed, and to verify the result with an ionic contamination test rather than with a visual inspection, in the same way as any other pcb cleaning process is validated.
Material Selection for the Mask
Standard liquid photo imageable mask is adequate for many assemblies, but where the mask must remain over a transmission line, a low loss material with a stable dielectric constant, of the kind covered by hf laminate selection criteria, is worth specifying. The improvement is modest on a short line and significant on a long one or in a resonant structure, and the cost difference is small compared with the cost of a board that has to be respun because a filter is off frequency.
Colour and finish also matter. A dark mask absorbs more heat, which affects the soldering of small joints, and a glossy finish reflects more, which can matter for optical inspection. Neither effect is large, but both should be considered when the assembly is being specified, because a change of mask colour between prototypes and production is a change of the electrical environment as well as a change of appearance. Recording the mask material, thickness and cure schedule alongside the stack-up is what allows the measured performance to be reproduced.
Mask and the Connector Launch
The transition from a coaxial connector to a microstrip line is the most performance critical region of a microwave board, and the mask matters there more than anywhere else. Mask that encroaches on the launch changes the capacitance at that point and shifts the reference plane of the measurement, which is why the mask opening around a launch is usually generous and the pad is left clean and coplanar with the surrounding ground.
Grounding is equally important. A connector that is soldered to a ground pad which is separated from the main ground plane by a narrow neck of copper will behave inductively, and no amount of care with the mask will compensate. The mask pattern should therefore be drawn so that the ground pad under the launch is large, is connected to the plane with vias immediately beside it, and is not crossed by a mask dam that also happens to be a solder dam. Getting the launch right is worth more than any adjustment downstream.
Verifying the Assembly
Verification of a microwave assembly combines electrical measurement and physical inspection. On the electrical side, a vector network analyser measurement of the structure, compared with the simulated response and with a bare laminate reference, shows whether the mask and the solder have shifted the response. Doing this on a coupon that carries the same mask treatment as the production board is the cheapest way to establish the offset.
On the physical side, a cross section through a test coupon shows the mask thickness over a conductor, the fillet of the solder joint and the condition of the mask at the edge of an opening. Taken together with an ionic contamination measurement after cleaning, these checks confirm that the process is producing the assembly that was designed, and they are the records that allow a change of mask material or of cleaning chemistry to be evaluated before it reaches production.
FAQ
Should the mask always be removed from a transmission line? Only where the impedance or the loss matters enough to justify the loss of protection. On a short line at a moderate frequency the effect is small and the mask is better left in place.
Can a mask be applied after the microwave substrate is assembled? Not reliably. The mask is part of the board fabrication, and applying it later cannot achieve a controlled thickness over a conductor.
How is a mask judged suitable for a microwave assembly? By its dielectric constant, its loss tangent, its cure behaviour and its resistance to the flux and solvent used in the soldering process. All four should be specified.
