Micro Coax Cable Stripping
Micro-coax cables are made up of a metallic conductor with a dielectric insulation which in turn is covered by a braided shield and a polymer jacket. There is no clear distinciton of what size constitutes a micro-coax, as opposed to an ordinary coaxial cable, but certainly once the conductor gets below 30 AWG, the cable presents difficulties in stripping.
Micro-coax cables can be used singly, within a larger cable assembly or ribbonized. Ribbonizing micro-coax cables is a popular design feature, as once stripped, the fixed pitch enables more convenient soldering to PCB.
Micro-coax is found in a wide variety of applications from mobile phones and laptops to medical cabling such as ultra-sound cables.
Alternatives to lasers
Single micro-coax down to 38 AWG can be stripped by sophisticated semi-automatic mechanical wire strippers. Rotating blades are precisely set to cut into the various depths of the cable to cut each layer sequentially. However for smaller wires, mechanical solutions are no longer possible and ribbonized micro-coax cannot be stripped mechanically either.
To strip micro-coax (either single coax or ribbonized coax) the following process steps are carried out in sequence:
- Cut jacket with carbon dioxide laser
- Cut jacket end is then pulled back to reveal the shield
- Shield is cut with a fibre laser
- Cut shield slug (still held in the jacket slug) is pulled back to reveal the dielectric
- Dielectric is cut with carbon dioxider laser
- Slug (including jacket, shield and dielectric) is pulled back to reveal the conductor
For ribbonized micro-coax, the slugs are held together maintaining the pitch of the wires for connectorization.
Microcoax prep (from left to right): jacket cut and pulled; shield cut; shield pulledl dielectric cut and pulled
The Mercury range of wire strippers is used for cutting the jacket and dielectric. The Mercury 1,2 and 3 can be used for this purpose, although the convenience of precision strip positioning along the coax of the Mercury 2 and 3 make them the preference.
A critical step is the cutting of the shield. The standard laser method for cutting the shield is to solder dip the cable so that that shields become tinned. In the tinning process, the individual shield wires bond together to make a solid mass. The fiber laser (Gemini 1,2 or 3) then cuts into the soldered shield. The process is set to make a deep cut into the shield, but not to cut through it. This creates a natural fracture line, so when the coax is bent at the stress point, the shield snaps cleanly. The shield is now pre-tinned for connecting an eath bar.
For the latest generation of flexible micro-coax cables, there may be insufficient shielding to form a solid mass when soldered. In this case an alternative is required. Solderless scribing is where the shield wires are directly cut with the fiber laser. As the shield is fully cut through and directly in contact with the dielectic, it is vital that the cutting process is controlled so as not to damage the dielectric. Laser Wire Solutions can advise on suitable materials for the dielectric, as this has a direct impact on the possibility of applying solderless scribing. Both the Gemini 3 and Titan systems are capable of solderless scribing.
Solderless scribing can be attractive even if there is sufficient shield for soldered scribing. A disadvantage of the soldered scribing method is that there are many individual process steps with specialised tooling needed between laser steps to pull back the slug and snap the shield. Adopting the solderless process means that a single laser automatic process can be used. Laser Wire Solutions is the only supplier to have developed an all-in-one automatic micro-coax stripping system. See the product details of Titan for more information.