Basic PCBs

The minimum structures have even improved in some cases, in no case have they worsened.
However, the derivation or presentation has changed:

• the copper thickness has to be considered and defined in a more differentiated way: For outer layers, following IPC-6012, the copper foil thickness before metallization is selected and no longer the final copper layer thickness.
• in addition to the STANDARD category, there is now another ADVANCED category.

Regarding the question it looks like this:

• previously: final copper thickness 70 µm - conductor width 125 µm and conductor spacing 160 µm (not possible in all plants, therefore also at that time de facto already ADVANCED)
• new: Copper foil thickness 1 oz. (34.3 µm) - conductor width 120 µm and conductor spacing in the ADVANCED category 160 µm (STANDARD 180 µm).

Inner layers can be structured simply by etching, the copper design is covered by resist for this purpose. In the process of outer layers, in addition to the pure structuring, the holes (PTHs) have to be metallized to establish the electrical connection between the copper layers. For this purpose, copper is deposited in the hole barrel and on the outer layers. For photo-structuring, the process must therefore be inverted, the holes and the copper design are open for copper buildup, and the resist prevents copper buildup in the places where there should be no copper in the end product.

This question is generally discussed very controversially, there are arguments for and against. From our experience, removal does not bring any significant improvements in terms of manufacturability and manufacturing costs. In addition, there is less anchoring and stability of the barrel, which can have a negative effect during thermo-mechanical stress, for example soldering cycles or temperature changes in field use. With RIGID.flex technology, the removal of non-used/non-functional pads on the polyimide layers must be avoided at all costs - otherwise reliability can be severely impaired!

Basically, in this 1 oz. to ½ oz. comparison (35 µm copper foil thickness to 17.5 µm), the thinner copper foil is less expensive. At the same time, smaller spacings are easier to produce with the thinner foil thickness, and the higher yield also has a cost-reducing effect.

The copper distribution should basically be as uniform as possible. For signal layers, the area share of copper is lower than for planes, but should be in the range of 40 to 60 %. Larger areas without copper should be filled and connected to a defined potential. For example, a high area copper content improves heat dissipation through heat distribution and heat dissipation. High uniformity of copper distribution is important for manufacturability with low thickness and etch tolerances and low board warpage.

IPC-2221 defines that the conductor width is measured at the base, see IPC-2221B Fig. 10.1. The manufacturing data for the photoresist are therefore modified accordingly with the aim of achieving the desired final width according to customer data after etching.

Prepreg (and copper foil) on the outside is easier and cheaper, the layer stack needs one core less. Prepreg and copper foil do not carry structures and therefore do not need to be registered to the inner layers. In the case of cores on the outside, their inner layers must be structured before pressing and registered for pressing, while the outer sides must remain completely covered with copper for the subsequent processes. The manufacturing process with foil lamination is therefore simpler than with core lamination.

Always the final diameter, only for press-fit holes the tool diameter is specified additionally.

With the standard process, the edge coverage is at least 5 µm. With multiple coating, the coating is thicker everywhere, and with s.mask technology, the coating thickness can be increased locally.

At Würth Elektronik, the material for prepregs and cores in one stackup is always of the same grade, i.e. BASIC Multilayer is made of FR4 from one manufacturer. The prepregs are made of the same material as the cores, but the resin is not fully cured ("B-stage"). As in the production of the core (from prepregs and copper foil), the prepreg resin is just fully cured (cross-linked, "C-stage") during multilayer production.

Yes, even then the conductors and copper structures should be protected from contact and contamination. So solder mask has a function not only for the soldering process, but also for the use of the PCB.