SMT007 Magazine

SMT007-Aug2019

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54 SMT007 MAGAZINE I AUGUST 2019 existing equipment set used for PCB drilling and fabrication. It is fast, has multiple sta- tions (4–6), there is no learning curve, and every PCB fabricator already has the equip- ment. Hole quality depends on the drill tool, and there are minimum hole-size limitations to mechanically machining. Laser Machining Requires a capital investment for a laser drill; however, the equipment will primarily be used for laser drilling the raw PCBs. Many PCB fab- ricators do not have existing laser equipment and the cost is much higher than mechani- cal machines. The actual lasering is faster per hole, but it is a single station. The laser pro- duces a precise hole and has virtually no mini- mum hole size. Flex Coverlayer or Flexible Solder Mask 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. However, this is not recommended when the design requires any dynamic or extreme flex- ing. The other option is a laminated flex cover- layer. These are typically materials that have a makeup that is identical to the flex core mate- rial and are best suited for dynamic flexible cir- cuit applications. The flex coverlayer material is a polyimide sheet with acrylic adhesive on one side. It is usually pre-machined to create openings in the sheet where the final finish is required. The coverlayer sheets are typically applied in a lamination press using special pads to ensure conformity around the copper features on the flex layer. For rigid-flex circuits, the coverlayer is generally 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. Fig- ure 1 shows an example of flexible solder mask and coverlayer being used in flex circuits. It is worthwhile to note here that the 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 cir- cuits are used in the construction of rigid-flex circuits where they serve in the role of bond ply. It is also important to note that coverlayer material can come in typi cal thickness intervals from 0.5–5 mils (12–125 μm) of polyimide and 0.5–3 mils (12–75 μm) of adhesive. Based on the design and application, the adhesive thick - ness requirement is typically decided by the copper thickness to which it is being bonded. The higher copper weight, the more adhesive is needed. There are various thicknesses of cov - erlayer coatings, and a general rule of thumb Figure 1: Flexible solder mask and coverlayer used in flex circuits.

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