Issue link: https://iconnect007.uberflip.com/i/1533085
70 DESIGN007 MAGAZINE I MARCH 2025 standard manufacturing methods to allow high yields. is unbalanced stackup can cause lower yields as it deviates from the beaten path. Also, this stackup can cause warping dur- ing and aer the SMT reflow process, as each material has a different Tg (glass transition temperature) and CTE (coefficient of ther- mal expansion) and may not meet the flatness requirement according to IPC-6013. As a flex circuit supplier, we need to lead the customer to the most robust, highest yield, lowest cost design. e stackup shown in Figure 2 will hold its flatness better through the SMT process and stays on the beaten path of manufacturability. Another example is conductive/non-con- ductive via fill on a multilayer flex circuit. I've had customers send data packages requesting quotes for a multilayer flex circuit or an unbalanced rigid-flex requiring via fill. e issue is that via fill plugging can only be done on rigid PCB boards and balanced rigid-flex in the rigid area only. e reason for this is that the surface of a circuit needs to be flat in order to planarize the panel. e process for via fill is: 1. Aer the panel is drilled and plated, a conductive or non-conductive material is squeezed onto the panel to fill the vias. 2. Aer via fill curing, the panel is then pla- narized, which is usually a barrel brush that brushes the cured excess via-fill material off the surface. 3. Once the surface is cleaned, a copper cap plating is applied. e process, again, can only be done on a flat surface. Multilayer flex circuits use either acrylic or epoxy thermally bonding adhesives that contour around inner etched surfaces, causing a non-flat outer surface topography and thereby making the planarizing process nearly impossible. I realized there was enough room on the board to move the via-in-pad to via outside the SMT pad. is eliminated the need for via fill. We could now stay with build- ing a multilayer flex rather than converting it to a more expensive rigid-flex circuit. e customer was very pleased with the sug- gestions, and we won the order. e last example is a customer that had a "flex to install" design. is means the circuit is flexed only during installation. Aer we built a small prototype run of 25 pieces, the customer noticed a few issues. 1. e customer was bending right at a stiffener and forming it by hand. Figure 2: Example of a balanced stackup.