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14 SMT Magazine • March 2017 This is done in a dry oven at low temperature for several hours. The intent is to evaporate all moisture that has been absorbed by the plastic and adhesive layers during storage. This is a big- ger problem during the summer months when atmospheric humidity is high. Less of an issue during the dryer winter months. However, no matter the season, if the moisture is not baked out, negative results are possible. When introduced to the high temperatures required to melt solder, 680°F to 750°F, the mois- ture trapped between the layers can boil quick- ly and cause the laminated layers to separate. Once the layers are separated the flexible circuit cannot be used reliably as it allows air into the circuit which contains water, that will eventu- ally corrode and render the circuit useless. It is a prudent first step in any circuit assembly pro- cess to bake out the moisture. Solder Choices for Assembly of Flexible Circuits Before the year 2000, most circuit assem- blies used solder that consisted of tin and lead. A popular ratio of the alloy was 63% tin and 37% lead. However, the European Union passed a directive named Restriction of Hazardous Substances, commonly known as RoHS, or Di- rective 2002/95/EC. It restricted the use of lead, mercury, cadmium and other substances in products sold there. Electronic industries world- wide were affected and had to come up with a substitute for the tin/lead alloys that had been used for decades. Today, both RoHS and non-RoHS solders ex- ist and are used. A typical RoHS compliant sol- der will contain no lead and be made instead of tin, silver and copper. This new solder requires higher temperatures to melt than the tin/lead versions and looks differently as well. Both RoHS and non-RoHS solders come mixed with flux that must be cleaned or flux that does not require to be cleaned. The flux in the version that requires cleaning is very cor- rosive and can be conductive if left on the cir- cuit, but is easily cleaned with water. The flux in no-clean solder leaves an inert clear residue that may remain on the circuit forever without adverse effects. The use of these solder options on flexible circuits is common and generally re- quires no special considerations, aside from the melting temperature. Hand or Manual Soldering Process The hand or manual soldering process re- quires a skillful assembler to attach components to a flexible circuit one solder joint at a time. A compliant solder joint, defined by the gov- erning body, IPC, makes no distinction for flex- ible circuits. The complications added to the solder process come from the thin flexible na- ASSEMBLY OF FLEXIBLE CIRCUITS Figure 1: Flexible circuit. Figure 2: Hand solder.