Issue link: https://iconnect007.uberflip.com/i/1276973
AUGUST 2020 I DESIGN007 MAGAZINE 73 Myth #3: Design Has the Final Say in Material Temperature Rating Johnson: A third myth concerns materials— specifically, who specifies the glass transition temperature, which will avoid delamination and warping issues. Dack: As an EMS provider, we often see a lack of definition in the designation of high-temp or low-temp materials. It is generally accepted that the best stakeholder to define material temp specification is the PCB designer. But this myth is not necessarily the fault of the designer. Designers are expected to specify things regarding electrical performance. But it is a mistake to presume PCB designers should be specifying manufacturing processes. There is wisdom in the old saying that designers need to specify what they want in the part, not how to achieve it. Many times, a PCB designer is called upon to design or specify PCB features for which he or she has no idea of where the part will be built or the materials, processing, or capabilities available. This is another good reason for designers and manufacturers to establish a means of communication prior to beginning the design. We see material on the fab drawing that reads "Material: FR-4." This specifies a fire-retardant glass epoxy material but is not very specific at all with regard to other important charac- teristics. When we order the boards, we have to make certain determinations for that board with regard to vulnerability to heat in order to be successful on our processing lines. In the customer's best interest, we want the PCB to be the lowest cost possible. We like to use the least expensive material available for pricing reasons; however, this reason can conflict with quality, which is also a prior- ity. When "FR-4" is listed as the only mate- rial specification on the bare board, we have to estimate how many thermal cycles the board will go through before it is fully assembled so it will not warp or delaminate. If overall manufacturing data is limited and we do not have the complete picture, the boards will be typically quoted with low-temp material not to drive up costs. But that strategy can backfire on us during the manufacturing review, which sometimes occurs after we win the quote. Sometimes, the board may have to go through several additional thermal excursions. In this case, we will determine that we need to pay for the higher temperature material. The thought process we go through involves asking, "Is the board a single-side two-layer board?" This makes it a candidate for low- temperature material because it's only going to see one thermal excursion, and it doesn't have to be flipped over and parts placed on the backside. If the panel warps slightly, it's insignificant. However, if the board is a two- sided assembly, multilayer, we have to look at the layer stackup and make sure that it's a bal- anced layer so that it won't warp. We may also consider paying more for the materials to pre- vent warping and delamination during more thermal cycles. A thermal cycle is a period of time that the board will be exposed to heat, which happens in a reflow oven, rework, and during wave sol- dering or select wave soldering. Any time this board is being exposed to heat, it can adversely affect the board laminate. For a double-sided, multilayer assembly, we will go with a higher- temp material—150–170°C—which will cost the customer more. If we go to even more layers and add another process, it may take a single pass to install top-side surface-mount components, a second pass to install the bottom side components, and then a third pass through a wave solder operation to install the through-hole compo- Example of a warped board.