PCB Stack-Up Principle

PCB stack-up has a great influence on the EMC (electromagnetic compatibility) performance of a product. A good stack can reduce radiation from the loops on the PCB and the cables attached to the board. On the other hand, a poor stack-up can increase the radiation from both of these mechanisms considerably.

To design multi-layer PCB circuit boards, designers need to confirm the circuit boards structure firstly according to the scale of circuit, the size of circuit boards, and the requirements of electromagnetic compatibility (EMC). That is to say, designers have to use 2, 4, 6, or more layers of circuit boards. If the design requires the high density ball grid array (BGA) devices, the minimal number of wiring layers must be considered. For years, people always think that the less PCB layers would spend the lower cost. However, many other factors affect PCB manufacturing costs. In recent years, the differences between costs of multi-layer boards have been reduced significantly. As soon as the number of layers been determined, the placement of the inner layer and different signals in these layers are decided --- this is the stack-up design of multi-layer PCB. Careful planning and rational stack-up designs beforehand will save a lot of efforts in the following wiring and future production.

After the number of circuit board layers determined, It needs to reasonably arrange the placement order of the circuit of each layer. In this part, there are two main factors to be considered. One is the distribution of special signal layers. The other is the distribution of power layer and ground layer.
The more layers circuit boards have, the more varieties arrangement on special signal layers, ground layers and power layers have. Thus, it is more difficult to choose the best combination method, but the general principles are as follows

(1) The signal layer should be next to an internal power layer (internal power/ground layer), shielded by the copper film of internal power layer.

(2) The internal power layer should be integrated with ground layer tightly. That means, the thickness of medium between internal power layer and ground layer should take the smaller value, improving the power supply capacitor between the internal power layer and ground layer, and increasing the resonant frequency. If the electric potential difference between internal power layer and ground layer is not important, a smaller insulation thickness can be used, like 5mil (0.127mm).

(3) To avoid the two signal layers directly adjacent. It is easy to introduce crosstalk between adjacent signal layers, leading to the fail of the circuit. To place a ground layer between two signal layers can avoid cross talk efficiently.

(4) Multiple grounded internal power layers can reduce the ground impedance effectively. For example, A signal layer and B signal layer use ground plane respectively can reduce common-mode interference effectively.

(5) The symmetry of layer structure.

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Printed circuit board (PCB) fixture test introduction

Rapidly increasing complexity of PCB assemblies makes visual defect detection nearly impossible. Receiving a PCBA without electrical verification can significantly delay design verification of prototypes and system integration of low volume PCBAs. The cost and time to develop fixture based in-circuit testing (ICT) is prohibitive in the prototype stage and the investment in ICT or functional test is tough to justify for low volume manufacturing. 

 

Vergent Products offers a solution – flying probe fixtureless testing based on parametric test of each component. Fixtureless test development is low cost and very fast. There is no costly fixture to design and build. Tests are generated from CAD information which enables a high degree of test coverage and flexibility. 

 

Testing your low volume products and prototypes allows for detection and repair of electrical defects such as wrong parts, reversed parts, missing parts, solder opens, and major solder defects. Fixtureless testing can help you improve your products time-to-market by achieving the highest possible quality levels early in the product life cycle. Fixtureless test allows your engineers to focus their efforts on prototype design verification instead of debugging process issues. 

 

Fixtureless testers use a flexible moving probe system. Multiple probes are automatically moved very rapidly to various test points on the assembly – hence the name: “flying probe”. These probes are capable of static DC voltage, current, resistance, capacitance, and inductance measurements. Test points are derived from CAD data to ensure accuracy and to maximize test coverage. 

    

Here is a brief description of some of the features of the Seica S40 has to offer: 

    1>Four independently moving topside contact probes; Test speeds up to 12 – 20 tests per second 

    2>Test area up to 16” x 20” 

    3>Positioning repeatability accuracy of +/- 0.12mils; Minimum positioning resolution of +- 0.20 mils in each axes 

    4>Minimum probe contact pitch of 0.12 mils between probe pins; Minimum pad size of 4.0 mils 

    5>Maximum component height of 0.9 inches including board thickness 

    6>Automated optical inspection, in-circuit programming, and boundary scan functional test capable (optional) 

 

For best turn around time on board test program generation and debug please provide: 

    1>Schematics 

    2>ASCII CAD data 

    3>Raw card 

    4>Populated board 

 

The test development NRE charges are very reasonable. 

The basic knowledge of printed circuit board (PCB)

Printed circuit board (PCB) almost appear in every electronic device. If there is electronic components in some kind of device, they also be mounted on various sizes of PCB. Except fixing these varieties of small sparts, the main function of PCB is to provide electrical connection for sorts of PCB parts to each other. As the electronic equipments become more complicate and require more spares, the circuits and spares on PCB also become more and more intensive. Standard PCB look like this. Bare board (without any top spares) is also often to be called Printed Wiring Board (PWB). 

The baseboard itself is made of insulate and inflexible materials. The tiny circuit material on the surface which can be seen called the copper foil. Oringinally copper foil is covering the entire board, but some parts of them were etched during manufacturing process, the left parts become tiny circuits like mash lines. These circuits are called conductor pattern or PCB layout, and used to provide electrical connection for PCB components.

For fixing the parts on the PCB, we weld their pins on the wiring directly. For a basic PCB (single-panel), all components were concentrated on the one side, and wirings were concentrated on the other side. As such, we need punch holes on the board, for pins to get through to the other side of the board, so pins of components were welded on the other side of the board. Because of it, the front side and back side of PCB are called Component Side and Solder Side.

If there is a certain part on PCB need to be removed or put back after the production is completed, socket will be used when the component mounting. Because the socket is soldered directly on the board, the parts can be any disassembly. See below is ZIF (Zero Insertion Force, zero-pluggable power type) socket, which allows parts (here refers to the CPU) can be easily inserted into the socket, also can be removed. Fixed pole near socket can be fixed after you inserted into the socket.

If you interconcet two PCBs, generally we will use the edge conncetor of known as "Goldfinger". Goldfinger contains many bare copper pads, in fact these copper pads are also one part of PCB routing. Normally, when connection we will put the gold finger from one PCB inserted into the appropriate slot from another piece of PCB (commonly called expansion slots). In the computer, such as graphics cards, sound cards, or other similar interface cards, are connected to the motherboard by the gold finger. 

Green or brown on the PCB is solder mask color. This layer is an insulating protective layer can protect the copper, the parts can be prevented from being welded to the incorrect place. Besides, a silk screen layer will be printed on the solder resist layer. Usually there will be printed some text and symbols (mostly white) to mark out the position of the parts on the board. The silk screen is also called legend.