PCBA Tech - SMT reel and bulk and microstrip transmission line

Tray and bulk materials in SMT chip substitution processing

When purchasing components for SMT chip processing, material packaging is very important! Most component distributors offer the same components in multiple packages to accommodate different pick-and-place loading preferences. SMT patch material packaging mainly includes: bulk materials, tray materials, pallets, tubes and batches. Each packaging type has its advantages, and it is difficult to determine which packaging type is best for a particular job.

Panel materials and bulk materials

The tapes and reels of bulk materials are transported to the pick-and-place machine through tapes containing the parts (usually small ICs). However, the main difference is the length of the tape. The "cutting tape" provides components in the form of small pieces of tape, while the "pan-packed material" is long and continuous, and is wound in the pan-packed material. Although their use depends on the type of board to be assembled, tray materials are usually the better and more common choice.

The biggest benefit of roll packaging is time. There is no need to load 20 separate tapes, and the reel only requires the operator to load the paper feeder once to carry out a continuous feeding.

In addition, quality standards require operators to notify quality control (QC) personnel every time a new component is loaded into the machine. According to lean principles, this is wasteful.

SMT tray materials can also enable operators to avoid paper jams. The cut tape sometimes gets stuck in the feeder, and the parts of the reel tend to avoid paper jams. However, when the circuit board only needs a small amount of a certain type of components, dicing tape is absolutely necessary. It is important to keep this in mind during the procurement phase.

Other common packaging

Although cut tape and reel packaging are often the most commonly used packaging, there are still many other types of packaging available. Let us briefly describe the other two options in order to make the best packaging decision for your specific production line.

Pallet material

Pallets are usually used for larger surface mount racks, such as QFN and BGA. Pallets require less wear, because larger components are much more expensive. Although fewer parts are usually used when transporting parts, more protection is provided in the use of pipes.

The purchase of smt patch materials is very important, a qualified board is composed of qualified materials, so when purchasing materials, you need to pay attention to the packaging method of the materials

Microstrip transmission line in RF PCB design

So far, microstrip is still the most commonly used transmission line structure in radio frequency and microwave design. However, as the speed and density of digital and hybrid technology designs continue to increase, the situation is becoming less and less.

Because for the same impedance, the microstrip line is usually wider than the strip line, and because the radiation associated with the microstrip line increases, it requires both more wiring space and larger distances for nearby traces. In pure RF or microwave designs, this is usually not a problem, but with the demand for smaller product sizes and the consequent increase in component density, it becomes a less easily available option.

Structure

The microstrip transmission line is composed of a conductor (usually copper) with a width of W and a thickness of t. The conductor is routed on a ground plane wider than the transmission line itself and separated by a dielectric with a thickness of H. The best practice is to ensure that the ground reference plane extends at least 3H on both sides of the surface microstrip trace.

advantage

· Historically, the main advantage of the microstrip line may be the ability to use only two layers of boards, while all components are mounted on one side. This simplifies the manufacturing and assembly process and is the lowest cost RF circuit board solution. Since all connections and components are on the same surface, there is no need to use vias when making connections. In addition to cost factors, this is also ideal, because the use of vias does not increase capacitance or inductance.

· For the same impedance, microstrip traces are usually wider than stripline traces. Therefore, since the etching tolerance in manufacturing is an absolute value, it is easier to control the characteristic impedance of the trace more strictly. Therefore, if your trace width is 20 mils, and the width is reduced by 1 mil by over-etching, then this is an Very small percentage change. For example, in FR408 material, a microstrip trace that is 20 mils higher than the ground and 11.5 mils higher, with a dielectric constant of 3.8, will produce approximately 50.8 ohms. If this trace is reduced to 19 mils, the characteristic impedance will be approximately 52.6 ohms, and the characteristic impedance will increase by 3.6%. In the same material, a 5mil stripline with 6 mils grounded up and down will produce about 50.35 ohms, but when reducing 1 mil to 4 mils, the characteristic impedance will be about 56.1 ohms, an increase of 11.5%. When completing certain designs, the characteristic impedance of the final trace is not specified, but the final width is specified. In the same over-etching scheme, reducing the 5 million traces of 1 million mils will reduce the final trace width by 20%, and reducing the 20 mil traces of 1 million mils will reduce the width by 5%.

shortcoming

· Since the microstrip transmission line is usually very wide and laid on the surface of the circuit board, this means that the surface area available for component placement will be reduced. This makes microstrip useless for high-density hybrid technology designs, which are almost always valuable for space.

· Microstrip transmission lines will radiate more than other transmission line types, which will be the main contributor to the overall radiated EMI of the product.

· Thirdly, as the radiation from the microstrip increases, crosstalk becomes a problem, so it is necessary to provide an increase in the spacing from other circuit components, which leads to a decrease in the available wiring density.

· Microstrip designs usually require external shielding, which increases cost and complexity. In fact, this has become one of the most important issues in the design of portable devices such as mobile phones. The driving force of many products is getting smaller and smaller, and therefore thinner and thinner. This means that the shielding layer will be closer to the surface of the circuit board, which will increase the capacitance per unit length of the transmission line, thereby changing its impedance. When choosing to use microstrip transmission lines and deriving impedance models, please consider carefully. If the trace needs to pass through an external shielding wall, it may be necessary to modify the transmission line width by a small distance, usually through a "tunnel", which is usually closer to the surface of the board than the top of the shield.

· The characteristic impedance of the microstrip will be affected by solder resist or other surface coatings. From one manufacturer to another SMT manufacturer, or even from one board to another board of the same supplier, the application of these coatings may be very inconsistent. Therefore, the impact of these coatings on the impedance of surface microstrip traces is very unknown.