Selecting PCB core thickness becomes an issue when a štampana ploča (PCB) manufacturer receives a quote requesting a multilayer design and material requirements are incomplete or not stated at all. Sometimes this happens because the combination of PCB core materials used is not important for performance; If the overall thickness requirement is met, the end user may not care about the thickness or type of each layer.

But at other times, performance is more important and the thickness needs to be tightly controlled for optimal performance. If the PCB designer clearly communicates all requirements in the documentation, then the manufacturer will know what the requirements are and will set the materials accordingly.

Issues PCB designers need to consider

It helps designers understand the materials available and commonly used, so they can use appropriate design rules to build PCBS quickly and correctly. What follows is a brief description of which material types manufacturers prefer to use, and what they might need to rotate work quickly without delaying your project.

Understand PCB laminate cost and inventory

It is important to understand that PCB laminate materials are sold and work in a “system” and that the core material and prepreg retained by the manufacturer for immediate use are usually from the same system. In other words, the constituent elements are all parts of a particular product, but with some variations, such as thickness, copper weight and prepreg style. In addition to familiarity and repeatability, there are other reasons to stock a limited number of laminate types.

Prepreg and inner core systems are formulated to work together, but may not work correctly when used in combination with other products. For example, the Isola 370HR core material will not be used in the same stack as the Nelco 4000-13 prepreg. It’s possible they’ll work together in some situations, but more likely they won’t. Hybrid systems take you into uncharted territory, where the behavior of materials (well known when used as homogeneous systems) can no longer be taken for granted. Careless or unwitting mixing and matching of material types can lead to serious failures, so no manufacturer will mix and match unless the type is proven to be suitable for “mixed” stacking.

Another reason to keep a narrow material inventory is the high cost of UL certification, so it is common in the PCB industry to limit the number of certifications to a relatively small selection of materials. Manufacturers will often agree to make products on laminate without standard stock, but be aware that they cannot provide UL certification through QC documentation. This is a good choice for non-UL designs if it is disclosed and agreed in advance and the manufacturer is familiar with the processing requirements of the laminating system in question. For UL work, it is best to find out the manufacturer inventory of your choice and design boards to match it.

Ipc-4101d and foil construction

Now that these facts are out in the open, there are two other things to know before jumping into design. First, it is best to specify laminates according to industry specification IPC-4101D and not to name specific products that not everyone can stock.

Secondly, it is easiest to construct multiple layers using the “foil” construction method. Foil construction means that the top and bottom layers (outer) are made from a single piece of copper foil and laminated to the remaining layers with prepreg. While it may seem intuitive to build an 8-layer PCB with four double-sided cores, it is preferable to use foil externally first, and then three cores for L2-L3, L4-L5, and L6-L7. In other words, plan to design a multi-layer stack so that the number of cores is as follows :(total number of layers minus 2) divided by 2. Next, it is useful to know something about core properties. Themselves.

The core is supplied in a fully cured PIECE of FR4 with copper plated on both sides. Cores have a wide range of thicknesses, and more commonly used sizes are usually stored in larger stocks. These are the thicknesses to keep in mind, especially when you need to order quick turnaround products so that you don’t waste the lead time of the order waiting for non-standard materials to arrive from the distributor.

Common iron core and copper thickness

The cores most commonly used to construct 0.062 “thick multilayers are 0.005”, 0.008 “, 0.014 “, 0.021, 0.028 “, and 0.039 “. Inventory of 0.047 “is also common, as it is sometimes used to build 2-layer boards. The other core that will always be stored is 0.059 in., because it is used to produce 2-ply boards that are 0.062 in. Thick, but can only be used for thicker multiply boards, such as 0.093 in. For this position, we limit the scope to a core design with a final nominal thickness of 0.062 inch.

Copper thicknesses range from half an ounce to three to four ounces, depending on the particular manufacturer’s product mix, but most stocks can be in two ounces or less. Keep this in mind and remember that almost all stocks will use the same copper weight on both sides of the core. Try to avoid PCB design requirements that require different copper on each side, as often this requires a special purchase and may require a rush charge (rush delivery), sometimes not even meeting the distributor’s minimum order.

For example, if you want to use 1oz of copper on an airplane and plan to use H oz of signal, consider making the airplane in H oz or increasing the signal to 1oz to make the core use both sides like copper with weight. Of course, you can only do this if you can still meet the electrical requirements of the design and have enough XY areas to accommodate widened trace/space design rules to meet the 1oz minimum at the signal layer. If you can meet these conditions, it is best to use it like a copper weight. Otherwise, you may need to consider a few extra days of lead time.

Assuming that you have selected the appropriate core thickness and available copper weight, various combinations of prepreg sheets are used to establish the remaining dielectric locations until the total thickness required is met. For designs that do not require impedance control, you can leave the prepreg option to the manufacturer. They will use their preferred “standard” version. On the other hand, if you do have impedance requirements, state these requirements in the documentation so that the manufacturer can adjust the amount of prepreg between cores to meet the specified values.

Kontrola impedanse

Whether impedance control is required or not, it is not recommended that you attempt to document the type and thickness of prepreg for each location unless you are proficient in this practice.Often, such detailed stacks eventually need to be adjusted, so they can cause delays. Instead, your stack diagram can show the core thickness of the inner layer pair and indicate “prepreg position required based on impedance and overall thickness requirements”. This allows manufacturers to create ideal laminations to match your design.

Profil

An ideal stack of cores based on existing stock is critical to avoiding unnecessary delays when ordering quick turns with tight timelines. Most PCB manufacturers use similar multilayer structures based on the same kernel as their competitors. Unless the PCB is highly customized, there is no magic or secret construction. Therefore, it is worth familiarizing yourself with the preferred material for a particular layer and making every effort to design a PCB to match it. There will always be exceptions for specific design requirements, but in general, standard materials are the best choice.