Standard Design Recommendations

Holes

Wirin g ho les Surf a c e mount a cc ess
Minco can drill through-holes as small as 0.0039" (0.10mm). A0.020" (0.50mm) through-hole size is typical. Standard finished hole tolerance is ±0.003" (0.08mm). For all circuits, the finished through-hole size should be 0.003" (0.08mm) to 0.010" (0.25mm) larger in diameter than the component lead. This depends on the number of leads per component, and the posi- tional tolerance of the component leads.It is best to specify round(instead of slotted), through- holes. This will reduce drilling time and cost. Minco can provide flex circuits with areas that are specificallydesigned for surface mount components. Because covers are drilled, not silk-screened, round access holes are easier to provide. Square access holes will add to cost because the pad access area would have to be punched out with a punch-and-die. Square pads with round access holes are a good compromise. Below are some ideas for configuring pads for surface mount.Photoimageable coverlay materials are also available, and canprovide intricate, irregular shaped openings for dense surface mount patterns. So l d e rin g tip s •Since polyimide absorbs moisture, circuits must be baked (1hour @ 120°C) before soldering.•Pads located in large conductor areas, suchas ground planes, voltage planes, or heat sinks, should be provided with relief areas, as illustrated. This limits heat dissipation for easier soldering.•When hand soldering pins in dense clusters, try not to solderadjacent pins one after another. Move around to avoid local overheating.•Minco can solder connectors or components (SMT or Thru-hole) as an added service.•Minco can supply circuits in panel form for easier componentassembly. Access Ho l e Access Ho l e Leads P a d s
Whenever possible, design pads larger than the access holes. Ifspace is critial, use hold-down tabs. Hold-down tabs are espe- cially important for single-layer circuits, because a single-layer circuit does not have the added strength of plated through- holes. A variety of hold-down tab designs are available. Thermal relief P a d fi lle t s
Pad fIllets improve etched yield and material strength. Filletsare appropriate when the pad diameter is greater than the con- necting strand width. Acute angles at the interface between con- ductors and pads are to be avoided by using fillets to minimize the concentration of stress at the interface. Before ll ets After ll ets 17