While no software is available, there are steps to determine how much is “just enough.”
Question: My company designs systems for unmanned aerial vehicles. Weight is a huge issue for us. Is there a rule of thumb or software tool available to calculate the weight of a flex or rigid-flex PCB?
Many of today’s electronics are weight-sensitive. Aerospace applications have a critical demand for the lowest possible weight and mass due to the high cost of sending any object into orbit. “Cost to orbit” can range from $5,000 to $14,000 per pound. These costs demand careful review of all materials making up the flex circuit. Extreme weight sensitivity is no longer a concern limited to aerospace engineers. The demand for “wearable” technology has accelerated dramatically, replacing aerospace as the lead in the race for the lightest possible flex. Even non-weight critical applications can benefit from lighter goods, which are reflected in lower shipping costs due to significant weight reductions.
That said, this is not an easy question to answer. In reality, I am not aware of any program or software that will calculate the weight of a flex or rigid board. There are ways we can obtain a rough idea of the finished weight that will give us a good ballpark to work with.
How do we go about these calculations? Starting with the basics, we know copper = 0.297 lb./in3 (8.23 g/cm3), FR-4 = 0.069 lb./in3 (1.9 g/cm3), polyimide = 0.051 lb./in3 (1.42 g/cm3), and acrylic = 0.040 lb./in3 (1.12 g/cm3). We can use these numbers to roughly calculate a final weight by taking the material thickness and total surface area of each layer in our stack-up. Fortunately, most CAD tools make this easier by giving us surface area, conductor length and width. We already know our height based on our material choices made upfront.
One of the most important things we can do to control weight is to review our material choices and design features with an eye toward “just enough.” Copper reduction is key, as it is clearly the heaviest component. Trace width depends on the amount of current the trace needs to carry. We don’t want to create an unsafe condition; however, in cases where weight is a real concern, we should provide only what is required.
The biggest potential for excessive copper weight that can be easily controlled is electrolytic copper plating. Panel plating the copper outer layers will likely more than double the outer layers’ weight. Using a pattern plate or “pads only” plating process will markedly reduce the overall weight without degrading system performance. Copper shields are also a big source of added weight; dropping our design from 1 oz. copper to 9˜µm will net huge savings with little or no impact on shield performance. Though we need to use care in how much material we remove, changing to a cross-hatched shield can drop our final weight further. For maximum shield weight reduction we can consider conductive films. These films also have the benefit of eliminating outer covers. This further reduces weight and makes them excellent options for EMF/noise suppression.
One area often overlooked is the final finish choice. HASL/solder of any type will have the greatest impact on weight. Clearly if the flex has any electrical components or connectors solder cannot be avoided, although its impact can be limited by gold plating exposed copper in areas not requiring components (ZIF, card-edge connections, etc.). The downside of this approach is the need to mask off these areas prior to HASL. Multiple surface finishes are possible but at greater financial cost.
Next in line for review is the choice of stiffener material and stiffener thickness. FR-4 is the most common material chosen for flex circuit stiffeners. It seems to be a default to call out 0.031" or 0.062" thick FR-4 on a drawing. There are times when the FR-4 thickness must be selected to meet a physical need, as in the case of a card-edge connector or ZIF end, in which case there is no choice. It is important to look at the application closely and ask if all that material is absolutely necessary. Often we can get away with using FR-4 stiffeners as thin as 10 mils. We can also consider using 5 mil polyimide film, which adds the convenience of panel-based layup, reducing not only weight but overall cost as well.
Flex design is always a contest between mechanical and electrical requirements; rarely does one complement the other. Reducing weight requires careful consideration of all aspects of flex design and careful balancing of opposing needs. If a lighter construction is the goal, flex is ideally suited for applications requiring lower overall weight and mass.