Surface install combined with flex PCB is in wide use throughout the PCB sector. Land pattern geometry for many typical surface install elements is well specified in IPC-7351 for stiff PCB board applications. Nonetheless, the land pattern geometry will certainly need a slight modification of conventional design policies when applied to flex PCB applications.
Making use of openings or slots drilled or directed into the coverlayer dielectric before lamination is a common means for flex PCB manufacturers to gain access to solder lands. Nonetheless, if traces are transmitted directly right into the land, misregistration of the coverlayer might cause the creation of an anxiety riser. Side or corner access to the land is much more forgiving to misregistration. The laser cut, mechanically punched or routed coverlayer opening can be made rectangular. Rectangular openings can also be attained by using photoimageable cover movies in place of a coverlayer.
SMT tool lands for both discrete as well as leaded gadgets need to be encompassed permit capture by the coverlayer. Normally lands should be 250 to 375µm (0.010″ to 0.015″) larger than the coverlayer opening provided for the land to promote land capture and avoid unwanted training of the land during assembly or repair service. Just like single-sided through-hole lands for flex PCB, the objective is to avoid land lift throughout soldering operations and also to give additional strength against element pull-away in operation. This is of higher importance with elements of higher mass. The offered examples of usual methods utilized to gain access to surface place attributes in flex PCB design applications while maintaining hold-down capacity.
The coverlayer openings for peripherally-leaded SMT lands can be accessed, either discretely or in a gang style, by prerouting or punching the coverlayer prior to lamination. Use of photo-imageable cover films make this activity a lot easier.
Except for the shared requirement for filleting, double-sided flex PCB circuits with plated through-holes do not require tie-downs because of the rivet effect from the plated through-hole. The rivet impact of the plated through-hole successfully avoids the pad from raising far from the surface of the flexible circuits during soldering processes at excessive temperature levels. Plated via holes are specifically recommended if quite small pads are required by the design to make certain development of a reputable solder joint. This could call for the addition of a second layer of copper, making a single-sided design a double-sided one. Ease of handling as well as increases in dependability might balance out any kind of increases in cost.
SWITCH PLATING
An alternative plated through-hole building and construction can be produced using a process called button plating. Button plating is a process where through-holes as well as vias are selectively plated with copper. The keynote is straightforward; however, it requires affordable treatment in the flex PCB manufacturing procedure to attain success. In practice, the flex PCB manufacturer initially drills as well as makes the hole walls of the flexible circuits conductive using an ideal modern technology. The rigid flex PCB manufacturer following takes the panel in for imaging where the panel is covered with a photo-imageable plating resist and also the holes as well as vias to be plated are exposed and created. A copper pattern plating step follows as the hole wall surfaces and the annuli of the holes are plated to the specified thickness. The very first resist is stripped away and also a second plating stand up to is used and also revealed to produce a positive circuit image which could then be etched to create a steel circuit pattern. The holes and vias are tented over in this procedure, avoiding the steel etching chemistry from entering the vias as well as etching out the holes, thereby developing electrical opens.