Question: Should I use polyimide coverlay, or flexible solder mask?
The fundamental purpose of either flexible solder mask or polyimide coverlay is no different than rigid solder mask. It is needed to encapsulate and protect the external circuit layers. But, when moving to a flexible circuit design the bending, folding and flexing requirements of the circuit in assembly and end use most often dictate which materials are recommended to perform that task.
Flexible Solder Mask
Flexible solder mask is very similar to the standard rigid board solder mask that we are all used to but formulated with an additional component in the material to allow it to flex and bend. Flexible solder mask is applied in a similar fashion to rigid board solder mask and can be less expensive than the film-based polyimide coverlay. Another benefit of this product is the ability to define tight feature areas.
Sounds like a perfect solution, right? The trade-off here to consider is that while the product is intended to be flexed, it is not as flexible as a film-based solution and will crack if over-exerted. For this reason, it is often selected for single and double sided flex applications with less than 2 oz copper and applications that that do not require a tight bend radius or will be dynamically flexing in final use.
Film Based Coverlay
If the end-use of the design is going to be dynamically flexing, or pushing limits of bend radius guidelines, film-based coverlay is recommended. Film-based coverlay is the most robust coverlay option and the most common material used for flex and rigid flex applications.
This option is constructed with a layer of polyimide and a layer of adhesive, with all kinds of different thicknesses available. The options for adhesive are ½ mil to 3 mils thick and the options for polyimide film are ½ mil to 5 mils thick. But, please keep in mind that specialized options may add cost and lead-time to the design.
The most common option, and the least expensive of the film-based options, is 1 mil polyimide with 1 mil adhesive. Reasons to deviate from the norm would include needing a thinner coverlay option to accommodate a tight minimum bend radius in end use or using thicker than 1 ounce of copper. The general rule of thumb is 1 mil of adhesive per 1 ounce of copper to ensure encapsulation. If you do need to deviate from the standard construction, it is a good idea to work on the stack up with your fabricator to understand what materials they may have in stock. This will help keep both cost and lead-time down.
I had mentioned earlier that this option is a little more expensive than the flexible solder mask. One piece of that is raw material costs, the other is the added processing involved with this option. The raw material is typically supplied in a roll form and cut to size. From there, the openings needed may be drilled, routed, punched, or laser cut depending on the complexity of the design and feature sizes. Once the pattern is created, the film is then aligned to the circuit layer and pressed under heat and pressure, over time, to cure the adhesive.
Drilling is the most common method to create the feature openings and there are few design related elements to keep in mind:
Drill bits are round, so the fabricator will be working with a round or oval size hole in the material and often these are covering square surface mount pads.
A larger annular ring will be needed to accommodate both the adhesive squeeze out that occurs during the pressing operation and manufacturing tolerances associated with drilling.
The minimum web thickness will also need to be large enough to ensure a strong enough adhesive bond and to avoid what has been termed “the swiss cheese effect”. Sometimes what looks perfectly acceptable on our large, over-sized monitors, doesn’t translate to fabrication. Anyone hanging around a flex fab shop long enough, myself included, has likely seen this phenomenon. Once drilled, the backer and entry material are removed and the coverlay has so many holes in it, it resembles swiss cheese and unfortunately is not stable enough to be registered and laminated to the circuit pattern.
So, what are the options available if your design requires tighter feature sizes than a mechanical drilling process can accommodate?
One extremely common solution to this is to “gang open” the coverlay. Essentially creating a larger, single opening that exposes multiple pads. Fabricators will often suggest this when they review designs that could be problematic with registration of the coverlay.
Another solution would be to create a punch die to stamp the coverlay instead of drilling. This will have tighter tolerances than a mechanical drill and allow registration to tighter features.
Yet another solution is to laser cut the coverlay. This could be done in the film itself and then registered to the circuit pattern. It could also be done in panel form after the polyimide film has been laminated to the circuit pattern. The laser then comes through and opens all the copper features that are intended to be exposed. This option, while more expensive than the other options, can accommodate extremely tight feature patterns.
Answering the original question:
Whether to use flexible solder mask or film-based polyimide coverlay really depends on how the flexible circuit is going to be used during installation and in the end application. Flexible solder mask can be a little less expensive and is an excellent option for low layer count flex, with one ounce or less copper, that is not going to see tight bending or folding, or dynamic flexing. Polyimide film, while slightly more expensive, is the most common coverlay option, the most flexible option, and is extremely robust for dynamic flexing applications and tight bending and folding requirements.
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