Pre-treatment and PTH Solutions for Flex-rigid PCBs - Sponsored Whitepaper

The demand for flex and flex-rigid boards of ever increasing complexity but at lower costs has never been as high as at present. To be able to compete in such a market requires reliable processes with the capability to handle diverse and emerging materials and designs. This paper will highlight some of the difficulties faced by flex and flex-rigid PCB manufacturers today and will provide working solutions and innovations. Desmear is a common request for flex-rigid materials to ensure good interconnection quality as well as to reduce the comparatively higher costs of plasma processing. The difficulties commonly found with desmearing different lay-ups of flex-rigid will be discussed as well as some useful tips. Pre-treatment is one of the key process steps for ensuring good flex material coverage and adhesion. Several electroless copper pre-treatment concepts as well as a direct concept will be discussed showing the benefits and results for even the most demanding of board layouts and materials.

Introduction Due to the different nature of flexible materials and the common requirement to use adhesives it means that manufacturing these boards causes issues not normally faced by standard FR4 PCB producers. The high water absorption, high Tg, chemical resistance and lack of copper adhesion for polyimide are major factors in the difficulty of producing flexible boards. Improvements in the processing of these materials and board types is required especially due to the large increase in flex and flex-rigid board requirements year on year.

Desmear For flex and flex-rigid boards containing polyimide the use of plasma etching for smear removal, adhesion enhancement and surface activation is long known. This technique, however, can be expensive, time consuming and not conducive to mass production requirements, especially for the competitive mobile phone board market. A lot of attention is being made to use permanganate desmear for these board types. The results vary wildly and depend a lot on material choices and build-ups.