Pad design specification
I. Pad naming rules
Pad naming should accurately convey the core process and physical properties. The structure is "assembly method + pad shape + through-hole shape + pad size + through-hole size", and code abbreviations are used to connect "process type → morphological characteristics → dimensional parameters" to ensure that the name conveys its meaning clearly and avoid ambiguity.
1. Assembly method: The core identifier for distinguishing welding processes
Represented by S (Surface Mount) and D (Through Hole) —— Surface mount does not require through - holes, and the pads only need to contact the surface of the component pins; through - hole insertion requires through - holes, and the pads need to surround the pins to provide soldering space. This is the underlying logic of pad design, which directly determines the subsequent structure (with or without through - holes) and process (reflow soldering/wave soldering).
2. Pad shape: The key code describing the physical shape of the pad
Use geometric feature abbreviations to correspond to different application scenarios:
CI (Circle): Universal pads, suitable for most regular pins.
OB (Oval, elliptical): Used when a larger solder-sucking surface or directional positioning is required (e.g., for heat-dissipating components);
SQ (Square): Enhance the symmetric mechanical strength (e.g., components with square pins);
RE (Rectangle): Suitable for strip-shaped pins or power components that require heat dissipation;
OC (Octagon): It combines the universality of a circle and the positioning accuracy of a square.
SH (Special Shape, irregular): Only used for special pins. Do not use it unless necessary (convert to regular shape first).
3. Through-hole shape: A unique attribute of through-hole pads
Surface mount pads have no through-holes, which are represented by NO; the shape of the through-holes of through-hole pads is logically consistent with the shape of the pads (for example, a circular pad corresponds to a circular through-hole CI, and an oval pad corresponds to an oval through-hole OB), which is directly related to the matching degree of pin insertion.
4. Pad size: Select the expression method according to the shape
D (Diameter): The diameter is used for circular pads, which is concise and intuitive.
LW (Length × Width): For non - circular pads such as rectangles and ellipses, use "Length × Width" to accurately convey the size boundaries.
5. Through-hole size: Key parameters of through-hole pads
Consistent with the logic of pad size (use D for circular through - holes and LW for rectangular through - holes), it directly corresponds to the clearance requirements for pin insertion (pin size + 0.2 - 1mm).
II. Core specifications for pad design
The essence of pad design is to balance soldering reliability, mechanical strength, and process compatibility. The following rules revolve around these three major goals, and each rule is supported by clear principles:
1. The shape must match the solder-sucking surface of the component pins
The "wetting" and "spreading" of solder depend on the contact area. Only when the shape of the solder pad is exactly the same as the tin-absorbing surface of the component pin can the solder maximize the coverage of the contact surface and reduce the risks of cold solder joints (incomplete wetting of the solder) and false solder joints (insufficient contact area). For example, rectangular pads should be used for rectangular pins, and circular pads for circular pins. This is the basis for the reliability of soldering.
2. Logic for adjusting the size of surface mount pads
The solder absorption surface of the surface mount pad should be 0.1 - 0.2 mm larger than that of the component pin (in normal cases), ensuring that there is sufficient space for the solder to adhere without causing solder overflow and short - circuit. In special cases, additional optimization is required:
Components with high heat generation (such as power resistors): Increase the pads in the heat dissipation direction by 1 - 1.5 mm —— Enhance heat conduction by expanding the area and quickly transfer the heat of the components to the PCB copper foil.
Heavy/large-volume components (such as large inductors): Increase the pads of the fixing pins by 0.5 - 1 mm —— to increase the mechanical strength and prevent the components from falling off due to their own weight or vibration.
3. Size formula and value-taking logic of through-hole pads
The through-hole pads shall meet the requirements of "smooth insertion + firm soldering". The formula is D = V + 2K (D = pad size, V = through-hole size, K = single-side soldering size). The core is to balance "processability" and "reliability":
V (Through-hole size): Pin size + 0.2 - 1 mm. For small components (such as 1/4W resistors), take 0.2 - 0.3 mm (thin pins, small gap to avoid difficult insertion); for large components (such as power transistors), take 0.5 - 1 mm (thick pins, large gap for convenient plugging and unplugging during maintenance) —— If the gap is too small, the insertion will be stuck; if it is too large, the solder will flow into the through-hole, resulting in a false solder joint.
K (Single-sided welding size): 0.3 - 2 mm. For small components, use 0.3 - 0.5 mm (to save space); for large components, use 0.8 - 2 mm (to increase the welding area and improve the mechanical strength). —— If K is too small, the welding strength will be insufficient; if it is too large, PCB space will be wasted.
4. Anti-short-circuit design of closely-pinned through-hole pads
For through-hole components with dense pins (such as high-density DIP chips), the pads on the non-soldering side need to be reduced or the copper removed, and oval pads should be used on the soldering side. Since the pin pitch is small (≤1.27mm), during soldering, the leaked solder tends to accumulate on the non-soldering side to form a solder bridge (short circuit). Reducing the pads on the non-soldering side can reduce the adhesion of solder, while the oval pads reserve a sufficient solder-absorbing surface on the soldering side, taking into account both reliability and short-circuit prevention.
5. Design of positioning pins for heavy/large-volume components
The positioning pins (without electrical connection) of heavy or large-volume components (such as large transformers) need to use through-holes without copper (without solder-absorbing surface). Copper pads will have an error of ±0.1mm during PCB manufacturing due to "copper foil etching + tin plating". Through-holes without copper are only determined by the drilling process (error ±0.05mm), and the positioning accuracy is doubled. At the same time, the absence of a solder-absorbing surface avoids the influence of solder flow on positioning during soldering.
6. Design of fixing holes with electrical connections
If the holes for fixing the PCB need to be grounded/powered, two circles of small vias should be drilled around the pads. Firstly, it is for "parallel conduction": the small vias are equivalent to increasing the conductive area and improving the efficiency of power transfer. Secondly, it is for "stress dispersion": the force of screwing during installation will be concentrated on the fixing holes, and the small vias can prevent the copper foil from cracking and maintain the stability of the electrical connection.
PCB Component Package Design Specification
I. PCB Package Naming Rules
The encapsulation naming should fully convey the component attributes and physical parameters. The structure is "component category + assembly method + encapsulation specification + number of pins + height + model". Use codes and numbers to connect "component type → process attributes → morphological characteristics → spatial parameters" to ensure that designers can quickly identify key information.
1. Component category: Represented by single-letter codes (Table I)
The core logic is "the component type determines the basic form of the package" - for example: the packages of C (capacitor) are mostly in sheet/columnar shapes, those of R (resistor) are mostly rectangular, and those of I (integrated circuit) are mostly multi - pin flat packages. Code correspondence:
- C = Capacitance D = Diode E = Anti-static/Lightning protection F = Fuse I = Integrated circuit J = Connector L = Inductive component M = MOS transistor R = Resistor S = Switch T = Transistor Y = Crystal oscillator
2. Assembly method: Consistent with the pads (S = Surface Mount, D = Through-Hole), which directly determines the pin structure of the package (no through-holes/with through-holes).
3. Packaging specifications: Represent specific forms with codes (Table II)
The logic is "basic encapsulation + modifiers", forming a hierarchical system:
- SMD = Surface Mount Device (General) SOP = Small Outline Package (Two-side leads, pitch 2.54mm) SSOP = Shrink Small Outline Package (1.27mm) MSOP = Medium Small Outline Package (1.5mm) QFP = Quad Flat Package (Four-side leads, pitch 0.635mm) BGA = Ball Grid Array (Solder balls at the bottom, high density) DIP = Dual In-line Package (Two-side leads, pitch 2.54mm)
4. Number of pins/Height: Represented by numbers (e.g., 16P5 = 16 pins, height 5mm), which directly determines the space occupation of the package (height needs to be reserved during PCB layout).
5. Model: Corresponding to the specific model of the component (e.g., "0805" resistor, "LM358" chip), ensure that the package corresponds to the component one by one.
II. Encapsulate the logic of the specification code
The core of the encapsulation specification code is to distinguish details by using "modifiers + basic encapsulation":
- Narrow pitch (SSOP): Reduces the pin pitch and is suitable for small-sized components.
- Wide pitch (MSOP): Increase the pin pitch, suitable for high-power components.
- Thin type (TSOP/TSSOP): Reduces the package thickness and is suitable for thin and light PCBs.
- Ball Grid Array (BGA): An array of solder balls at the bottom, which solves the problem of crowded pins of high-density chips.
- Direct Insertion Package (DIP): A traditional package suitable for components requiring high mechanical strength.
The design goal of these codes is to enable designers to quickly determine the "dimensions, pin spacing, and form" of the package through the codes without referring to detailed documents, thereby improving the design efficiency.
In summary, the design specifications for pads and packages all revolve around the three core aspects of "process adaptation + reliability + manufacturability". Each rule is supported by clear principles - pads focus on "soldering and mechanical strength", while packages focus on "component matching and space utilization". Ultimately, the "efficient, reliable, and mass-producible" PCB design is achieved.
SIP (Single In-line Package): Analysis of Standardized Parameters
SIP (Single In-line Package) refers to single in-line package. It is a classic through-hole/surface-mount compatible packaging form in the field of electronic components. Its core feature is that the pins are linearly arranged along one side of the component, combining mechanical stability and layout flexibility. It is widely used in scenarios such as industrial control, consumer electronics, and military equipment. Its parameter system constructs a standardized language for the entire link from design to mass production through six dimensions: functional classification, process type, physical specifications, number of pins, space limitations, and unique identification.
1.1 Component category: The first-level identifier of functional attributes
The component category is the functional classification code of components under the SIP package, which is directly related to their electrical characteristics, internal structure, and application scenarios. In essence, it uses standardized symbols (as shown in Table I) to quickly answer the question "What is inside this package?"
- If the code is `R`, it means the package contains resistors (such as metal film resistors and carbon film resistors), and the internal structure has a ceramic/metal substrate as the core.
- If the code is `C`, it represents a capacitor (such as aluminum electrolytic capacitors, ceramic chip capacitors), which contains an electrolyte or a dielectric layer inside.
- If the code is `IC`, it represents an integrated circuit (such as an operational amplifier or a microcontroller), and its interior consists of silicon wafers and metal interconnections.
This category is the "first filter" for circuit design. For example, when designing a power supply circuit, one should preferentially select the `L` (inductor) type SIP package; when designing a signal conditioning circuit, one should focus on the `IC` type SIP package. The standardized classification in Table 1 ensures the consistency of component selection in the supply chain and avoids mistakes like "installing a capacitor in a resistor package".
1.2 Assembly method: The core distinction of connection processes
The assembly method refers to the type of connection process between SIP components and PCB (printed circuit board). A two - digit code is used to clearly indicate "how to assemble", which directly determines the production efficiency and mechanical reliability.
- Code `D`: Through-Hole Mounting. The pins need to pass through the through-holes of the PCB and be fixed by wave soldering or manual soldering. The advantages are high mechanical strength and good resistance to vibration/impact, making it suitable for applications in harsh environments such as industrial robots and automotive electronics.
- Code `S`: Surface Mount Technology (SMT). The pins are directly attached to the surface pads of the PCB and soldered by reflow soldering. - Features: small size, high assembly efficiency (suitable for automated production lines), and suitable for the high - density layout of consumer electronics such as mobile phones and computers.
Note: Some SIP packages are compatible with two processes, but the main process must be clearly indicated in the code (e.g., `D/S` means the through-hole process is the primary one and the surface-mount process is the secondary one) to avoid process confusion during production.
1.3 Packaging specifications: The ID card of physics and technology
The encapsulation specification, which is a comprehensive set of physical parameters for SIP, integrates three key dimensions and serves as the "core input" for PCB design.
1. Dimension parameters: Include the length of the package body (it increases with the number of pins) and the width (the width of single - in - line packages is usually fixed at 1.27mm or 2.54mm).
2. Pin characteristics: Pin pitch (standard 2.54mm, narrow pitch 1.27mm), pin shape (round pin/flat pin, which affects welding conductivity);
3. Auxiliary design: Such as the positioning grooves on both sides of the body (to prevent reverse insertion), the heat dissipation pads at the bottom (for high-power devices), and the thickness of the gold-plated layer on the pins (which affects oxidation resistance).
Its alphabetic codes (as shown in Table II) need to correspond to the above parameters one by one. For example, code `A` corresponds to "2.54mm pitch, 10 pins, with positioning slot", and code `B` corresponds to "1.27mm pitch, 8 pins, without positioning slot". Such codes are the "design basis" of the PCB footprint library, directly determining the pad positions, via sizes, and layout pitches.
1.4 Number of pins: An intuitive reflection of functional complexity
The number of pins refers to the quantity of functional pins on one side of the SIP package, which is represented by three - digit Arabic numerals (if the number is less than three digits, add leading 0s. For example, 5 pins are written as `005`, and 12 pins are written as `012`), and the unit is "piece".
- The number of pins is directly related to the functional complexity of the component. For example, an 8-pin (`008`) component may be a simple voltage comparator, and a 20-pin (`020`) component may be a microcontroller with ADC (Analog-to-Digital Conversion).
- The number of pins determines the layout length of the PCB: With a 2.54mm pitch, a 10-pin SIP package requires a PCB length of 25.4mm (10 × 2.54mm).
It should be noted that non-functional pins such as positioning pins and heat dissipation pins are not counted in the number of pins to avoid confusing functions with auxiliary structures.
1.5 Height: A rigid indicator of space limitations
The height is the maximum vertical dimension of the SIP component (the distance from the bottom of the pins to the top of the package), which is represented by four digits + the letter `P` (`P` replaces the decimal point, and zeros are added to make it two decimal places), and the unit is "millimeters (mm)".
- Rule example: Write 1.5mm as `1P50`, 3mm as `3P00`, and 0.95mm as `0P95`;
- Significance: Height is the red line for product space design. For example, the height of SIP components in the mobile phone battery compartment cannot exceed the battery thickness (usually ≤ 4mm); the height of panel-type SIPs in industrial equipment cannot exceed the opening size of the chassis.
This parameter directly affects the "thinning" ability of the product and is one of the core constraints in consumer electronics design.
1.6 Model: Identification and traceability of unique functions
The model number is the unique functional identifier of SIP components, used to distinguish different products under the same packaging specification - in essence, it indicates "what the contents in the package are":
- If the package contains specific functional devices (such as a 10KΩ resistor or an LM358 operational amplifier), the model shall be represented by the code defined by the manufacturer (such as `10K` or `LM358`).
- If the package is a general carrier (e.g., a blank ceramic substrate, an unencapsulated lead frame), it shall be represented by `0000`.
The model is separated from the previous parameters by the `-` symbol (e.g., `R-D-A-008-1P50-10K`, where `10K` is the resistor model). It is the "core field" of the material code. During production, the manufacturer, batch, and specifications can be quickly traced through the model to avoid the risk of "wrong materials".
The parameter system of SIP essentially uses standardized language to connect the design end and the manufacturing end: Design engineers can quickly select components through parameters, production engineers can define processes through parameters, and quality engineers can trace problems through parameters. Understanding the "meaning boundary" of each parameter is the key to mastering SIP packaging applications.