At the conclusion of our webinar, Flex and Rigid-Flex PCBs: Technical Issues In Data Sets, we had several questions submitted to our presenter, Zachary Walker, Product Manager of Flex and Rigid-Flex Circuits at Epec. We have compiled these questions into a readable format on our blog.
Q&A From Our Live Webinar
Quick Links:
- The last time I ordered FPC from Epec, they came in a panel. What callout do I need to have to make sure they are de-paneled?
- Can Epec provide a copy of IPC-6013?
- Solid tab or v-scoring?
- Are all polyimide flex materials equivalent?
- What if a square inside the corner is required?
- How do we determine what are effective stiffeners for a specific design?
- When designing a rigid-flex design, do all the rigid portions need the same stack-up thickness?
- When designing a rigid-flex design, do all the rigid portions need the same stack up?
- When using combination soldermask and PI, how much room do you allow for an overlap of the 2 materials (assuming that transition occurs on stiffener areas)?
- Are these specs and best practices captured in a design guide on Epec's site?
- Do different color polyimide materials have different properties (e.g. dielectric strength)?
- For the high-speed material, what speeds are we talking about?
- What is the use case for a stainless-steel stiffener?
- What's the mechanical lifetime (i.e. number of flexes) for a flex board?
- Do you know what are the most common errors during assembly of flex cable for manufacturers?
- How many times can a flex cable be bent?
Watch the Recording Below:
Question: The last time I ordered FPC from Epec, they came in a panel. What callout do I need to have to make sure they are de-paneled?
Answer: The best way to call out panelization or no panelization is by marking it on the fabrication drawing. In some instances, we’ve had customers do so through their sales rep in an email, but we prefer putting it on the drawing so that it is easier to find and doesn’t get lost going from one person to another.
Question: Can Epec provide a copy of IPC-6013?
Answer: Unfortunately, we can't provide any copies of IPC specifications. These specifications are copy-protected. That makes it to where we can't share our copy of the specification. We generally prefer to guide people toward the IPC website where they can then purchase the IPC-6013 specification.
Question: Solid tab or v-scoring?
Answer: For flexible sections of rigid-flex and for flexible circuits, we do solid tabs. V-scoring cannot be performed for flex or rigid-flex designs. V-scoring is done with the intention to leave approximately the middle third of the stack-up to hold the board in place during the process to then be snapped away later after assembly. Typically, the centermost portion of the stack-up is a flexible material, which means that the board would not snap away and would require additional routing or cutting to separate from the array panel, defeating the purpose of the original v-scoring.
Question: Are all polyimide flex materials equivalent?
Answer: So, all available standard flex materials meet IPC specifications and are UL-certified. There are special cases and unique materials, such as one from DuPont that are rarely used in the industry or very certain use cases. For instance, for the LF series from them, we'll see it's a non-UL-certified legacy material, but we see those typically with qualified military medical designs where they can't readily change the design.
We'll see high-temperature or HT polyimide-based adhesive coverlays. Those operate on a 200-degree Celsius continuous operating temperature, very, very specialized material. Or there's a material, a TK material, which is Teflon polyamide hybrid. That's a high-speed material that's again in name, high speed/used for high-speed applications.
Question: What if a square inside the corner is required?
Answer: A non-plated hole is typically put at that corner, approximately 40 mils at the center of that inside corner to help create that sharp edge. It's never going to be perfect. It's hard to get square edges with round lasers or round drill bits. But we can get pretty close by doing that non-plated hole there at the corner. It'll eliminate any of the remaining radius and material from extending from the outline once the part is then finally routed to shape.
Question: How do we determine what are effective stiffeners for a specific design?
Answer: That is kind of a tricky and open question. Because each design is unique to itself, each design has specifications and requirements. So, some designs we'll see that don't require any stiffeners whatsoever, while some require multiple stiffeners like an FR-4 stiffener here and a polyimide stiffener there.
Because of this, we end up having to go on it on a case-by-case basis and review in a call or from the drawing to see what the requirements are to then determine what effective stiffeners are and if we have any recommendations for the stiffeners. So, good examples of that would be your component connector areas where we'll use an FR-4 stiffener, for instance, ZIF areas, which I had mentioned in the slide earlier, which use a polyimide.
And then other special use cases where you might need a stainless-steel stiffener, aluminum, and things of that nature.
Question: When designing a rigid-flex design, do all the rigid portions need the same stack-up thickness?
Answer: Yes. It is very strongly recommended that all rigid areas have the same thickness. Rigid areas of different thicknesses are very difficult to manufacture, if at all, and are very costly.
Question: When designing a rigid-flex design, do all the rigid portions need the same stack up?
Answer: Yes, all rigid sections must same the same stack-up as they are manufactured simultaneously.
Question: When using combination soldermask and PI, how much room do you allow for an overlap of the 2 materials (assuming that transition occurs on stiffener areas)?
Answer: In general, the overlap is 0.020”, but this may change depending on the specific configuration of the SMT pads.
Question: Are these specs and best practices captured in a design guide on Epec's site?
Answer: Yes, we have a design guide available on our website, however, it covers the basics only. Due to the large number of design elements possible, we recommend to our customers to allow Epec to review and provide specific design information at an early stage in your design development.
Question: Do different color polyimide materials have different properties (e.g. dielectric strength)?
Answer: Yes, the properties are the same no matter what color.
Question: For the high-speed material, what speeds are we talking about?
Answer: In general, 20 GHz and higher.
Question: What is the use case for a stainless-steel stiffener?
Answer: A stainless-steel stiffener is used when the available thickness for the stiffener is very limited, but a high degree of rigidity is required. Typically, stainless steel stifferner is 0.008” thick.
Question: What's the mechanical lifetime (i.e. number of flexes) for a flex board?
Answer: The answer will depend upon whether the application is a static fit or dynamic flexing.
For a static fit, the number of bends is typically in the 1-5 range depending upon the flex thickness and how close the bend radius is to the minimum bend capabilities of the flex construction.
For a dynamic flex application, the layer count is limited to 1 to 2 layers, and the minimum bend radius is 100x the flex thickness; this will allow for an infinite number of bends.
Question: Do you know what are the most common errors during assembly of flex cable for manufacturers?
Answer: Lack of or insufficient pre-bake. The pre-bake is required to remove all moisture from within the flex circuit to prevent delamination during the re-flow portion of assembly.
Question: How many times can a flex cable be bent?
Answer: The answer will depend upon whether the application is a static fit or dynamic flexing.
For a static fit, the number of bends is typically in the 1-5 range depending upon the flex thickness and how close the bend radius is to the minimum bend capabilities of the flex construction.
For a dynamic flex application, the layer count is limited to 1 to 2 layers, and the minimum bend radius is 100x the flex thickness; this will allow for an infinite number of bends.
