Three methods for incorporating non-heating leads into a heater pattern.

A flexible heater is needed where Constantan or Inconel is used for the heating area. Is there a way to incorporate a resistive layer into a standard flex circuit to have non-heating (copper) leads going to the heater pattern?

Answer: Yes, this is a common request and can certainly be done. Several methods are used to accomplish this, with limitations. A couple of these methods require selective plating of copper onto the resistive alloy element traces to lower the resistance, or using plated vias on other layers to make the non-heating connections. This works well on copper/nickel alloys like CuNi 715 or Constantan, but not so much so on Inconel 600 (these are the three most common resistive alloys used in flexible heater construction). The Inconel restriction is covered in more depth later in this column.

Know what details to include – or not include – in the flexible circuit drawing.

Since our most recent column covered drawing notes for flexible circuits, this may be a good time to go over many of the other features a good flexible circuit drawing should include, and maybe a few things to avoid.

Along with the drawing notes, the rest of the drawing features help define the details of the part as well as some of the acceptability requirements.

For starters, drill tables are important to help describe via structures. The manufacturer needs to know which holes are which. If there are filled vias and/or via-in-pad-plated-over (VIPPO), for instance, they are identifiable with a flag note, or segregate filled from non-filled holes on the table.

Fabricators need to understand your via strategy. Is this a simple through-hole construction, or are buried, blind and/or microvias employed? Sometimes we see blind vias overlapping layers; for example, one blind via from layer 1 to 8 and another from layer 12 to 6. Sounds cool on paper (or in CAD), but we can't do that. The fabricator can help you align the via choices with cost and reliability.