Cost modeling of PCB

this article I will cover what is required to build up the cost model of PCB. I will first guide you through the simple introduction of what PCB is and their corresponding production process in order to identify the cost elements; moreover, the major cost drivers. Please be noticed that PCB is a hard topic and requires decades of hands-on experience to have a thorough understanding of the technology and the application. The cost modeling is referred to based on my understanding and experience in server/switch, consumer electronics, and PC/NB in the following session. I encourage anyone to comment on my article or share the suggestion to sharpen the knowledge of cost modeling in PCB.?

A Printed Circuit Board (PCB) is multilayers of fiberglass core and resin coated laminates with electronic circuits consisting of thin strips of copper between the layers, plus plated holes for interlayer connections. The surface of the boards are finished with a paint-like solder mask (usually green) with pre-defined openings on copper pads, and then coated with either organic or metal finishes to prepare it for component assembly (PCBA). It is the basis of what we refer to as our motherboard (for servers and switches). PCB is a customized part due to the different needs of circuit layout and design. It can also be seen as one of the key PCBA components and one of the most important cost factors of overall PCBA cost.??

PCB cost modeling:?

PCB introduction is in the appendix A. There are a number of factors that drive manufacturing costs. In general, the more you require by size, layers, design requirements, and complexity, the more costly it will be. Let me use a simple example here. For networking switch products, PCB layers are usually 20 or higher and require ultra low loss material for high speed capabilities with low Dk and Df requirements. For server motherboards, PCB layers are usually 12 to 20. Other standard technology is below 12 layers. I address the major cost drivers of PCB due to material, production process, and design below.

Except for those cost drivers, PCB price is also highly sensitive to the annual/monthly volume and batch size. Supply base is another key driver. In the past decades, China has formed the lowest cost ecosystem in producing PCB. If PCB supply comes from Taiwan, the price is around 20-25% higher, 25-30% higher in Korea, and at least 50-100% higher in Japan.?

Following I will use a wireless board (Two layers with Plated Through- Hole and Surface Mount Components and surface mount on both sides) and work on the cost modeling of PCB step by step. Be noticed that the procedure will be different depending on the system/software used.?

Step 1: We will need to navigate the zip file which contains all of the gerber files. There are plenty of softwares in the market that can do the PCB pricing simulation. You may also build your own system to do the job.

Step 2: Identify your supply base because the selection will impact information such as labor rates, overhead cost, SG&A, etc. You may also get the information from your current PCB suppliers.??

Step 3: Input the estimated annual/monthly demand volume and the batch size.?

Step 4: PCB dimension in this case is 86.36mmx54.51mm. The manufacturing panel in stock is 609.6mmx457.2mm. Therefore, these boards will be cut out into the four segments and each segment produces 2 by 3 PCBs. By using this panel, we can produce 24 PCBs and the utilization rate is only 39.87%.?

Step 5: Input the configuration of PCB such as, PCB material type (This case is 2PT), core insulation type (This case is FR4), Glass transition temperature (This case is x<150Tg)

Step 6: Software or system will generate the necessary production procedure with available machine type (Be noticed it may NOT be optimized). Let’s use Drill circuit holes as an example. System will do the sorting of a common hole group with the same diameter and hole count. Therefore, machine rate and drilling cycle time can be applied.?

Each production procedure links to either labor or machine or both. It contains labor time (sec), cycle time (sec), batch setup time (sec), and overall how many parts can be produced in an hour as well as estimated yield. Assume the procedure is “Drill circuit hole”. The smallest production unit is a panel not a PCB. The number of boards per panel is 24. One should pay attention to either panel or PCB. Under this “Dril circuit hole” procedure, it specifies the number of simultaneous drilling heads possessed by the machine, the number of unique drill machine setup required in order to drill all the holes being processed in the current layer processing cycle, and part load/unload time. Under the part load/unload time, there are three operations: Load bottom protective plate time per panel, load top protective plate time per panel, and load panel time. Part unload time follows the same methodology and doesn't specify here. ? ? ?

Step 7: Till now, the system is capable of generating a cost summary, such as Material cost, labor cost, OH, amortized cost, SG&A, estimated margin, and final unit cost.?

Step 8: Remember that we don’t optimize the machine in step 6 but only select the available machine. Now it is the time to select not only available but also the most cost-efficient machine.

No matter the PCB cost modeling is done by your own system or the third party software, it is critical to have the timely data maintained, and maximize the reuse in modeling. Procurement manager can enjoy the benefit in doing the fact based negotiation with PCB suppliers and designer and product manager can do the scenario analysis for the unbuilt PCB. For example, the cost impact of adding an ENIG surface finish.?

Last but not the least I will use the last session to address the importance of cost impacted by design through deep diving the cost factors into 2 layers PCB and 4 layers PCB. These two PCBs use the same FR4 material, and same size/dimension.?

In 2 layers PCB case, the highest cost in mass production is plating and finishing (35%). Material is the second highest cost (15%). Photo etching and soldering mask and silkscreen are the third and share 12% each. The most efficient way to reduce the cost is to reduce the number of components and vias which means reducing the complexity. In 4 layers PCB case, material cost takes 25%, internal layers takes 20%, and then plating and finishing 17%. Reduce complexity means a detailed review of the internal layer design and component usage and target to reduce internal layers (20%) and plating and finishing (17%). When the PCB layer changes to 6, 8, or higher, the weight of design complexity will be more and more important to both cost and quality. ?

Appendix A: PCB introduction

The PCB production starts with various sheets of CCL (Copper Clad Laminate) material, and after multiple series of image transfer, development, chemical baths, stack-up of the various layers of laminate, mechanical pressing, drilling, copper plating, surface finish and then contour to size. I use the multilayer PCB production process as an example below:? Engineering→ Sheet Cutting→ Inner Film Lamination→ Inner Etching→ Inner Film Stripping→ Lamination→ Drilling→ PTH→ Panel Plating→ Dry Film Lamination→ Exposure & Developing→ Pattern Plating→ Tin-Lead Plating→ Stripping→ Etching→ Tin-Lead Stripping→ Solder Mask Coating→ Legend Printing→ Surface Finishing→ NC-Routing→ Electric Tests→ QI.

In the top of Supply Chain, glass suppliers which spin the glass from sand weave the glass into fabrics. They are provided to laminate suppliers which use the glass fabrics to go through treaters which they soak it in resin and cure it based on need. These sheets are either made into CCLs by cladding copper foil on each side or made into Prepreg sheets.??

The main materials used to construct the PCBs are generally called “Laminates”, and mainly two kinds are used, CCL and Prepreg. A CCL uses glass fabric as reinforcing material, soaked in resin and has copper foil cladded to both sides of the sheet. The CCL is the basis of an inner-layer sheet for circuitry designs. The Prepreg is the same glass fabric reinforced resin sheets, but without the copper foil. Prepregs go in between inner layer CCLs after the circuitry process is complete, and act as dielectric layer in the stack-up of each of the different layers.

Copper foil is used on the CCL and is also used as the very outer layer after the stack up of the different innerlayers and Prepreg are complete. The whole stack-up will go into a hot press process called “lamination” where the resin within the different layers will melt, bond and then cure to create the multi-layer board for further processing.

For the PCB supply chain material point of view, the very top is the glass suppliers, which spin the glass from sand (silicon) and also weave into fabrics. They provide to the Laminate suppliers, which use the glass fabrics to go through treaters which they soak it in resin and cure it according to need. Then these sheets are then either made into CCLs by cladding copper foil on each side, or made into Prepreg sheets. The curing process is different and I won’t go into more detail here.

PCB suppliers procure the CCL and Prepregs according to stack up designs (material spec, dielectric thickness, chemical and physical properties…) and different panel sizes for optimal material utilization to meet customer product performance specs and cost requirements.

Appendix B: IPC 6012 standards: Qualification for printed boards. Class 2 v.s. 3

Class 3 requires some special process steps which consume more production time and resources than Class 2.??

1. All panels are serialized to ensure full traceability
2. Plating times are 25% longer (1mil min vs. 0.8mil min)
3. Additional in-process micro-sectioning is completed in order to ensure compliance with Class 3 requirements for copper and annular ring.