PCB Tech - PCB pad design knowledge

Land, the basic unit of surface mount assembly, is used to form the land pattern of the circuit board, that is, a variety of land combinations designed for special component types. There is nothing more frustrating than a poorly designed pad structure. When a pad structure is not designed correctly, it is difficult, sometimes even impossible, to reach the expected solder joint. There are two English words for pad: Land and Pad, which can often be used interchangeably; however, in terms of function, Land is a two-dimensional surface feature used for surface mountable components, while Pad is a three-dimensional feature used for The components of the plug-in. As a general rule, Land does not include plated through-holes (PTH, plated through-hole). Bypass holes (via) are plated through holes (PTH) that connect different circuit layers. Blind vias connect the outermost layer with one or more inner layers, while the buried vias only connect the inner layer.

As noted earlier, the land usually does not include plated through holes (PTH). The PTH in a land land will take away a considerable amount of solder during the soldering process, resulting in insufficient solder joints in many cases. However, in some cases, the component wiring density is forced to change to this rule, most notably for the chip scale package (CSP, chip scale package). Below 1.0mm (0.0394") pitch, it is difficult to route a wire through the "labyrinth" of the pad. Blind bypass holes and microvias (microvia) are created in the pad, allowing direct wiring to another layer .Because these by-pass holes are small and blind, they will not suck too much solder away, resulting in little or no effect on the amount of tin in the solder joints.

There are many industry documents from IPC (Association Connecting Electronics Industries), EIA (Electronic Industry Alliance) and JEDEC (Solid State Technology Association), which should be used when designing the pad structure. The main document is IPC-SM-782 "Surface Mount Design and Land Structure Standard", which provides information about the land structure for surface mount components. When J-STD-001 "Requirements for Soldering Electrical and Electronic Assembly" and IPC-A-610 "Acceptability of Electronic Assembly" are used as solder joint process standards, the pad structure should meet the intent of IPC-SM-782. If the pad deviates greatly from IPC-SM-782, it will be difficult to achieve a solder joint that meets J-STD-001 and IPC-A-610.

Component knowledge (ie, component structure and mechanical size) is a basic requirement for the design of the pad structure. IPC-SM-782 widely uses two component documents: EIA-PDP-100 "Registration and Standard Mechanical Shape of Electronic Parts" and JEDEC95 publication "Registration and Standard Shape of Solid and Related Products". Indisputably, the most important of these files is the JEDEC 95 publication, because it handles the most complex components. It provides mechanical drawings of all registrations and standard appearances of solid components.

The JEDEC publication JESD30 (also available for free download from JEDEC's website) defines the abbreviations of components based on the characteristics of the package, materials, terminal location, package type, pin form, and number of terminals. Characteristic, material, location, form and quantity identifiers are optional.

Package feature: A single or multiple letter prefix that identifies features such as pitch and outline.

Packaging material: A one-letter prefix to identify the main packaging material.

Terminal location: A single letter prefix that confirms the terminal location relative to the package outline.

Package type: A two-letter mark that indicates the package shape type.

New pin style: a single letter suffix to confirm the pin style.

Number of terminals: a one-, two-, or three-digit number suffix to indicate the number of terminals.

A simple list of surface mount package feature identifiers includes:

· E to expand the spacing (>1.27 mm)

· F fine pitch (<0.5 mm); limited to QFP components

· S shrink spacing (<0.65 mm); all components except QFP.

· T thin type (1.0 mm body thickness)

A simple list of terminal location identifiers for surface mount includes:

· Dual pins are on opposite sides of a square or rectangular package.

· Quad pins are on the four sides of a square or rectangular package.

A simple list of surface mount package type identifiers includes:

· CC chip carrier package structure

· FP flat pack package structure

· GA grid array package structure

· SO small outline package structure

A simple list of surface mount related pin format identifiers includes:

· B A straight shank or spherical pin structure; this is a non-compliant pin form

· F A straight pin structure; this is a non-compliant pin form

· G A wing-shaped pin structure; this is a compliant pin form

· J A "J"-shaped bent lead structure; this is a compliant lead form

· N a leadless structure; this is a non-compliant lead form

· S An "S"-shaped pin structure; this is a compliant pin form

For example, the abbreviation F-PQFP-G208, description 0.5

mm (F) plastic (P) square (Q) flat package (FP), fin-shaped pins (G), number of terminals 208.

Detailed tolerance analysis of components and board surface features (ie, pad structure, reference points, etc.) is necessary. IPC-SM-782 explains how to perform this analysis. Many components (especially fine-pitch components) are designed in strict metric units. Do not design imperial pad structures for metric components. Accumulated structural errors produce mismatches and cannot be used for close-pitch components at all. Remember, 0.65mm is equal to 0.0256" and 0.5mm is equal to 0.0197".

In the IPC-SM-782 standard, each component and the corresponding pad structure are organized in four pages. The structure is as follows:

The first page contains general information about the component, including applicable documents, basic structure, number of terminals or pins, markings, carrier packaging format, process considerations, and soldering resistance.

The second page includes the component dimensions necessary todesign the land structure. For other component information, refer to EIA-PDP-100 and 95 publications.

The third page includes the details and dimensions of the corresponding pad structure. In order to produce the most suitable solder joint conditions, the pad structure described on this page is based on the maximum material condition (MMC). When using the least material condition (LMC, least material condition), the size may affect the formation of solder joints.

The fourth page includes tolerance analysis of components and pad structures. It also provides details on what to expect from the formation of solder joints. The strength of solder joints is affected by the amount of tin. Before deciding not to use a land structure based on MMC dimensions, tolerance analysis and solder joint evaluation should be carried out.