Flexible Circuits,Flexible printed circuit boards, Flex PCB, FPC Manufacturer,Rigid-Flex PCB

RayMing Flexible Circuits

CALLING OUT MATERIALS FOR FLEX CIRCUITS AND GETTING IT RIGHT

CALLING OUT MATERIALS FOR FLEX CIRCUITS AND GETTING IT RIGHT
Only a limited number of material types are used in flex circuit
manufacturing; however, materials and material callouts are the source
of many phone calls between designers and fabricators, mostly for
the purpose of seeking clarification. The following are some of the top
recurring issues.
Missing Dielectric Material Callouts
Flexible and rigid-flex circuits are manufactured using numerous types
of materials to meet a wide array of cost targets and performance
requirements, both physical and electrical. Because of this variety,
relative to the prospective concerns related to each choice, it is vitally
important that the designer provide detailed information about the
dielectric materials to be used. It is recommended that designers educate
themselves about the choices available in terms of cost and performance.
The Internet is packed with easily tapped information about flexible
circuit materials and how they might be used. The PCB fabricator can
also help with this topic. The basic flex material types are:
• Adhesiveless materials, which have no acrylic bonding the
copper to the polyimide dielectric
• Adhesive materials, which have acrylic bonding the copper to
the polyimide dielectric
• Flame retardant and non-flame retardant laminates,
coverlayers, and bond plys
Figure 1 illustrates the difference in adhesiveless flex cores vs. adhesive
flex cores.
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Figure 1: Adhesiveless vs. adhesive flex.
Each of the above have specific uses dependent on the end use needs.
However, if designers are uncertain as to what their options are and
what material might be best, they can always contact the PCB fabricator’s
engineering staff and ask for suggestions and recommendations.
Incomplete or Insufficient Rigid-Flex Base Material Type
Definition
The base material chosen defines the performance limits of the rigidflex
circuit in process and in field operation in many applications. With
most lead-free solders, the upper temperature excursions required for
soldering can be as high as 260°C, which normally mandates the use
of polyimide laminates. However, the material choice and its electrical
properties can affect other performance issues. One key area is in the
management of characteristic impedance of the circuit, and assurance
of signal integrity, with higher-frequency circuit designs becoming ever
more common (these latter subjects will be given more attention later).
Temperature range requirements for the rigid laminates used in rigidflex
constructions must also be considered and addressed. The rigid
material should be high-temperature capable. Polyimide glass laminate
is a common callout, but available improved epoxy resins are often
suitable.
Copper Type and Thickness Callout
While a number of different metal foils are available for making flexible
circuits, copper is the most commonly used metal for making electronic
interconnections. It is highly conductive, malleable (making it both flexible
and foldable), relatively easily processed by etching and plating, and
ADHESIVELESS FLEX CORE
1 OUNCE COPPER
ADHESIVELESS POLYIMIDE FLEX 1 MIL
1 OUNCE COPPER
1 OUNCE COPPER
ACRYLIC ADHESIVE
ADHESIVELESS POLYIMIDE FLEX 1 MIL
ACRYLIC ADHESIVE
1 OUNCE COPPER
FLEX CORE WITH ADHESIVE
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relatively inexpensive. The type of copper used most often for flexible
circuits is rolled and annealed copper (RA copper), which has the best
properties for dynamic flex applications. When designing the copper,
type and thickness choice should match the electrical and mechanical
requirements for the application.
Thicker copper is typically used for higher-power applications, and thinner
copper for circuits that require repeated bending (dynamic flexing). There
are many choices of copper thickness, but the most commonly used in
the creation of flexible circuit laminates are presently ½ ounce (17µm or
0.7mils) and 1 ounce (35µm or 1 mil).
Often additional copper may be plated onto the circuit, and this should
be accounted for in the specification as well. If the designer is uncertain,
he should contact engineering support for guidance.
Coverlayer or Solder Mask Over Flex Circuits
Coverlayers, or cover coats, are polymer materials used to cover and
protect the copper traces of the flex circuit product. As implied, there are
a number of different options available for protecting the circuits, and
they serve different design requirements in terms of cost, performance,
and flexural endurance optimization. When specifying the choice, it is
important to call out not just the type of coverlayer material but also
the thickness requirement. This can be very important in certain types
of constructions, especially when a flex circuit will experience dynamic
flexing during use. 
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In terms of cost, a flexible solder mask is generally the least expensive.
Some one- or two-layer flex circuits, that will not be subject to multiple
flex cycles or extreme radius bends, can be coated with an epoxy-based
solder mask that is designed to flex without cracking. This, however, is
not recommended when the design requires any dynamic or extreme
flexing. The other option is the laminated coverlayer. These are typically
materials that have a makeup that is identical to the flex core material’s
and are best suited for dynamic flexible circuit applications.
The coverlayer material is a polyimide sheet with acrylic adhesive on one
side. It is typically pre-machined to create openings in the sheet where
the final finish is required. The coverlayer sheets are usually applied in
a lamination press using special pads to assure conformity around the
copper features on the flex layer. For rigid-flex circuits, the coverlayer is
typically cut to only protrude into the rigid portion by no more than 50
mils. The purpose of this is to allow all the plated holes in the rigid-flex
to be void of any acrylic adhesive, as it can affect the hole wall plating
integrity.
Figure 2 shows an example of flexible solder mask and coverlayer being
used in flex circuits.
Figure 2: Flexible solder mask and coverlayer used in flex circuits.
It is worthwhile to note here that bond ply used to laminate flex layers
together is like a coverlayer, but it has adhesive on two sides. It is further
worth noting that prepregs (glass cloth which has been pre-impregnated
with a thermosetting resin), used for making rigid circuits, are used in the
construction of rigid-flex circuits where they serve in the role of bond ply.
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It is important to note that coverlayer material can come in typical
thickness intervals from .5 mil – 5 mils (12-125 µm) of polyimide and
.5 mil – 3 mils (12-75 µm) of adhesive. Based on your design, the adhesive
thickness requirement is typically decided by the copper thickness that
it is being bonded to. The higher copper weight the more adhesive is
needed. The same holds true for bond ply. Figure 3 illustrates a fourlayer
flex circuit and demonstrates the use of coverlayer and bond ply.
Figure 3: Four layer flex construction with bond ply and coverlayer.
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