PCB design is the key link contained in SMT technology, an important element determining SMT manufacturing quality. This article will analyze PCB design elements influencing its quality from the perspective of SMT equipment manufacturing. PCB design requirements from SMT manufacturing devices mainly include: PCB pattern, size, location hole, clamping edge, MARK, panel ways etc.
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• PCB pattern
In the automatic SMT production line, PCB production starts from loader and completes production after printing, chip mounting, soldering. Finally, it will be generated from unloader as a finished board. In this process, PCB is transmitted on the pathway of device, which requires that PCB pattern should be in accordance with pathway transmission between devices.
Figure 2 shows the standard rectangular PCB whose pathway clamping edge is as flat as a line so that this type of PCB is suitable for pathway transmission. Sometimes right angle is designed into chamfer.
For the PCB design in Figure 3, its pathway clamping edge isn't a straight line so that both PCB location and transmission in device will be influenced. The open space in Figure 3 can be supplemented to make its clamping edge be a straight line as in Figure 4. Another method is to add crack edge to PCB, which is shown in Figure 5.
• PCB Size
PCB design size has to conform to the maximum and minimum size requirements of printer and chip mounter. Up to now, the size of most devices is in the range from 50mmx50mm to 330mmx250mm (or 410mmx360mm).
If the thickness of PCB is too thin, its design size shouldn't be too big. Otherwise, PCB deformation will be caused by reflow temperature. It's ideal that length-to-width ratio is 3:2 or 4:3.
If PCB size is less than the minimum size requirement of equipment, panelization should be carried out. The number of panels is determined according to PCB size and thickness.
• PCB location hole
SMT location methods are divided into two types: location hole together with edge location and edge location. However, the location method applied in our company is Fiducial Mark.
• PCB clamping edge
Since PCB is transmitted on the pathway of device, components mustn't be placed along the direction of clamping edge, or components will be pressed by device, influencing chip mounting. Take the PCB in Figure 6(a) for example, some components are placed near the lower edge of the PCB, so the upper and lower edges mustn't be taken as clamping edges. However, there are no components near the two side edges, so the two short edges can be used as clamping edges, which is shown in Figure 6(b).
• Mark
PCB Mark is identification point for all full-automatic devices' identification and location used to modify PCB manufacturing error.
a. Shape: solid circle, square, triangle, rhombus, cross, hollow circle, oval etc. Solid circle is the first choice.
b. Size: the size has to be in the range from 0.5mm to 3mm. Solid circle with diameter of 1mm is the first choice.
c. Surface: its surface is the same as soldering plane of PCB pad with soldering plane being even, neither too thick nor too thin and excellent reflector effect.
A background zone should be arranged around Mark and other pads, silkscreen and solder mask can't be contained in the background zone, which is shown in Figure 7.
Figure 8 displays an excellent MARK design method while Figure 9 displays some MARK designs that are unreasonable.
Silkscreen characters and silkscreen lines are arranged around MARK in Figure 9, which will influence the device's identification of MARK and will lead to frequent alarm by MARK identification with manufacturing efficiency badly influenced.
• Panel method
In order to increase manufacturing efficiency, multiple small PCBs with the same or different shapes can be combined to form a panel. For some PCBs with double sides, top side and bottom side can be designed into one panel, which will produce a stencil so that cost can be reduced. This method also helps to reduce shift time for top side and bottom side, increasing manufacturing efficiency and device utilization.
The connection method of panels includes stamp hole and V groove, which are shown in Figure 10.
One requirement of V-groove connection method is to maintain the rest part of the board (uncut) equal to one fourth to one third of the thickness of the board. If too much of the board is cut off, the cut groove will possibly break down by the high temperature of reflow, leading to the falling of PCB that will be burnt in the reflow oven.
PCB design is such a complex technology that both device requirements and components layout, pad design and circuit design must be taken into consideration. Excellent PCB design is the significant element ensuring product quality. This article brings about some problems the PCB design should consider from the perspective of SMT manufacturing. As long as sufficient attention is paid to these problems, full automatic SMT manufacturing of SMT device can be carried out.
What is SMT?
Surface mount technology (SMT) is the process wherein components are mounted onto the surface of a printed circuit board. The components are designed specifically to be directly mounted, rather than hardwired, onto the circuit board for the vast majority of electronics.
A Surface Mount Technology (SMT) machine represents the latest generation of high-tech equipment assisting PCB manufacturers in resolving SMT assembly challenges. These machines streamline SMT assembly processes by employing fewer components, improving performance, and mitigating lead resistance and inductance issues.
This article will explore the various types and features of an effective SMT machine.
What do SMT machines do?
SMT machines are employed in the manufacturing of surface mount printed circuit boards (SMT PCBs). They encompass soldering equipment, component placement machines, and ovens.
These machines enable PCB companies to incorporate additional features onto smaller boards without relying on through-hole wire leads for component connections. Additionally, SMT equipment facilitates the printing and placement of elements on both sides of a PCB.
Types of SMT Machines
The process of placing electrical components on a PCB involves up to 6 different machines. Consequently, the input from all these machines collectively determines the output of an SMT machine.
Below is a breakdown of the various types of SMT machines:
-Pick-and-place machines:
These machines come in various types, including:
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- Manual pick-and-place machines:
This is the most widely used and cost-effective SMT machine for modular PCB assembly. An SMT operator performs the picking and placement procedure manually on this machine. Overall, it's an excellent choice for unsupervised high-speed operations.
- Semi-automatic pick-and-place machines:
Unlike the manual version, this type features advanced computer interface technology and vision assistance, making the operator's job easier.
- Automated picking and placement:
Fully automated pick-and-place machines incorporate picking and placement centering methods to enhance SMT productivity.
- SMT Curing Oven:
The curing oven is responsible for curing adhesive and baking solder paste. The baking process depends on the production volume of the machine.
- Screen Printer for Solder Paste:
The screen printing machine applies solder paste onto the PCB through screen printing. Typically, this process occurs prior to the assembly process by an SMT Machine Operator.
- Reflow Soldering Machine:
This machine facilitates the flow of solder from surface mount devices (SMD) to the Printed Circuit Board (PCB).
- Solvent Cleaning Equipment:
After soldering, there may be excess solder or flux residue. Solvent cleaning equipment is used to remove these residues.
- Inspection Equipment:
This machine detects and addresses any flaws found on the PCB during the inspection process.
SMT Feeder:
Improvements in feeder technology generally impact PCB assembly capabilities and production volume. SMT feeders are available in various shapes and sizes, depending on the packaging used for different mount components. For example:
Tape Feeder:
Tape feeders feature advanced capabilities, making them suitable for high-precision placement machines.
Tray Feeder:
Tray feeders are available in single-layer and multi-layer structures.
Stick Feeder:
Stick feeders and vibration feeders assist in loading components into the mounter.
Tube Feeder:
Tube feeders ensure accurate entry of all components into the placement head.
How to Choose the Best SMT Machine?
Before selecting an SMT machine, several factors should be taken into consideration:
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Part Size:
The size of the electronic components you intend to place plays a crucial role. Ensure that the machine's size matches the size of the components.
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Machine Accuracy:
Accuracy is a key factor in choosing an SMT machine. Look for machines with a standard accuracy of 0." in Surface Mount Technology.
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Machine Repeatability:
In addition to accuracy, repeatability is essential for consistent performance. Opt for machines with programmed software solutions that ensure maximum output.
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Placement Speed:
Consider the speed at which the machine picks up, checks, and installs electronic components. Look for machines that meet the IPC standard for rapid placement.
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Prices and Cost:
Choose a high-speed pick-and-place machine that offers features corresponding to its price. Ensure that the machine's capabilities align with your budget and requirements.
Summary:
SMT machines streamline work processes, reduce expenses, and boost production capacity. Before purchasing an SMT machine, it's essential to understand its operation, the various types available, programming procedures, and its key qualities.
We trust that our article has been helpful. If you have any further inquiries or wish to delve deeper into the topic of SMT machines, please don't hesitate to reach out to us. Our team is dedicated to addressing any questions you may have.
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