CircuiTree - September 2008 - (Page 45)

[ Ask the Flexperts ] By Mark Finstad You Don’t Have to Strain Yourself Over Marking Issues Is a bead of strain relief required at every rigid flex interface on a rigid flex circuit? Strain relief is a very common feature on rigid ﬂex circuits. The most common material for this application is a semi-rigid epoxy. Because the epoxy stays somewhat ﬂexible, it provides a nice transition between the rigid material and the ﬂex material and reduces the chance for a tear to form in the ﬂex material. The edges of rigid material Figure 1 Nicely Formed Strain Relief Bead on Thicker Hardboard Material the same thickness as the rigid board and then taper to zero thickness as shown in Figure 1. The overall bead width should be approx 0.050 in to 0.075 in wide along the entire length of the bead. However, when the rigid material is very thin (e.g., 0.005 in), it is virtually impossible to apply the strain relief bead and achieve the desired taper. The epoxy is generally dispensed with a needle on the end of a syringe or dispenser. To apply a functional strain relief bead to a 0.005 in hardboard, the dispensing needle would have to be approximately 0.010 in in diameter. Even if you could ﬁnd a dispensing needle that small, there is no way you would be able to squeeze the high viscosity epoxy through it. The smallest needle that you can push epoxy through is approximately 0.020 in diameter. If you were to use a needle this size to apply the strain relief to a 0.005 in hardboard, it would look like Figure 2. As you can see, this results in a bead that is semi-cylindrical. Because the bead does not have a uniform taper, it really has limited functional value as a strain relief. When marking flex circuits, do you need to use a special flexible ink? Figure 3 Ink Marking Was Used to Locate a Bend in the Flex. Because the Circuit Was Being Bent Sharply and the Marking Was on the Outside of the Bend, the Ink Cracked. This in Turn Caused the Polyimide Cover to Crack, Which Exposed the Underlying Conductors Figure 2 When the Hardboard Material Is Very Thin, the Bead Starts to Take on a Semi-Cylindrical Shape, Which Has Limited Value As a Strain Relief such as polyimide, BT, or FR-4 epoxy can be very sharp and can act as a shear on the ﬂex material if any type of twisting force is placed on the ﬂex arms. The epoxy strain relief bonds to both the ﬂex and rigid material and greatly reduces the shearing forces. A strain relief bead can also be used to protect the ﬂexible areas of a single-, double-, or multilayer ﬂex circuit with stiffeners. The bead would be placed along the edge of the stiffener just as it is done on a rigid ﬂex. For the strain relief bead to do what it is designed to do, it must have a nice uniform taper. It should start at approximately Legend and identiﬁcation info is marked on a ﬂex circuit with the same type of ink that would be used on a rigid PCB. And just like a rigid PCB, it can be applied via stamp, screen, or inkjet printer. It is more critical to evaluate the location of the marking than the type of ink or method of application. Most legend marking inks are reasonably ﬂexible due to the fact that they are applied in a very thin layer. However, these inks do not tolerate stretching well, so placing them on the outside of a very tight bend can cause problems. When a ﬂex circuit is bent sharply, the polyimide cover on the outside of the bend will stretch. If there is marking in that area, the marking ink will crack. When this happens, there is a chance that the cracks in the ink may propagate through the polyimide cover (Figure 3). For this reason, it is a good idea to keep all marking out of an area that will be formed. Etched text is another method of marking identiﬁcation on your ﬂex circuit if you have enough room to do it. When placing etched text, it is even more important to keep it out of bending areas. The added copper from the text can make those areas somewhat stiffer than the area surrounding it. Lines of text tend to create a stress concentration zone between the lines. Even a single line of text can create stress points between characters if the text is run perpendicular to the bend. Stress concentration points will invariably lead to a failure at some point. The best advice on marking is to use whatever material or process works best for your manufacturer because that will be the most cost effective. But when you are generating your gerbers, make sure that whenever possible you keep all text well away from the bend areas. ■ The Flexperts are Mark Finstad and Mark Verbrugge of Minco. Email: Mark.Finstad@minco.com Listen to Mark’s latest podcast at www.circuitree.com/podcast circuitree.com • September 2008 45 http://www.circuitree.com/podcast http://circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - September 2008

CircuiTree - September 2008 Contents My Line Industry Review Tech Talk Flexible Thinking New Halogen-Free Materials: Their Time Has Finally Arrived Asian Section IPC Issues PCB and Package Convergence Ask the Flexperts Market Outlook IPCA Showcase Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - September 2008

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