5 Major Requirements Design of PCB Surface Mount Technology

For manufacturing companies, one of the main requirements for an order is its manufacturability. Unfortunately, in practice, we have to face a situation when the received order is not technological. This leads to an increase in the complexity of its manufacture, and, consequently, to an increase in cost.

Pre-Designing Helps in Identifying Faults

A similar situation can occur both in the production of products of own design and in contract production. In some cases, non-technology should detect after the order does go into production. Although its identification is more of elimination, it should even detect at the design stage of the product.

To implement this task, it is necessary that the designer has an idea of the production possibilities, and most importantly, of the limitations that are inherent to him.

What we shall read in this article?

The article describes the practical experience of the joint work of the production and design department to create a document with the requirements for the design of printed circuit assemblies for automatic installation. The use of such a document makes it possible already at the stage of the pilot batch to get into production a product suitable for installation on an automatic line.

Title
Abbreviations:
PP - printed circuit board
SMD-components
Surface mounting Components
THT (Through Hole Technology)
Hole mounting technology (Outlet mounting)
THT-components

Outlet mounting components

Geometric parameters of the workpiece

The permissible thickness of the finished printed circuit board is from 0.6 to 3 mm.

It is recommended to make rectangular printed circuit board blanks with a width-to-length ratio of no more than 1:3.

The location of reference marks in the field of the printed circuit board

For this arrangement, there may be no technological field or the presence of a technological field of the minimum possible width of 4 mm.

The minimum width of the technological field depends on the location of global reference marks on the board and on the method of processing the edges of the workpiece.

Local reference marks:

Local fiducials can place for components with a pitch of 0.65 mm and less (BGA, QFP, CSP, QFN, connectors, etc.). Fiducials are located on opposite edges of the component outside zone setup.

With limited free space use, one reference sign between adjacent components; or may place fiducials inside installation zones under the component body.

In the absence of free space, only two local reference marks per single printed circuit board are allowed. In this case, they should be located diagonally at the maximum possible distance from each other.

The minimum distance between adjacent reference marks is 10 mm.

Location of components

If possible, the SMD components should be placed on one side of the PP.

If it is impossible to use one side of the PP for SMD installation, the components should be maximally focused on one of the sides.

SMD components with a height of more than 5 mm should, if possible, be located on one side of the PP —

The one that is as saturated as possible SMD components.

THT components should place on the side, if possible PP, maximally saturated with SMD components.

It is not allowed to have SMD components under components

The minimum gap between the contact pads of adjacent SMD components should be 0.4 mm

The minimum gap between the pads of SMT components in cylindrical housings (MELF, LL-34, etc.) and the pads of neighboring components in other housings is 1/2 the width of the contact pad for the cylindrical housing.

If there is a lack of space on the board, it is allowed to leave a gap between the contact pads of components in cylindrical housings of at least 0.6 mm.

Polar components (especially TNT components) should, if possible, be positioned with the same orientation.

Tracing elements of pp

The distance between the edge of the printed circuit board and any element of the printed circuit board (conductor, contact pad, hole, etc.)

Should be:

For scribing – 0.6 mm;

0.5 mm (as an exception – 0.35 mm in agreement with the head of the PP tracing department);

0.8 mm (if necessary, to use drilling, it is necessary to coordinate the possibility of its use with the head of the PP tracing department)

The contact pads must completely open with a mask

(nonsolder mask defined pad), while the gap between the mask and the pad should be 50 … 200 microns (for BGA microcircuits — strictly 50 microns).

The design of the Multiplicated blanks

The animation should carry out by direct transfer of printed circuit boards without turns. Another variant of animation must previously agree with the technological service.

When forming blanks, the preferred method of separating the boards is scribing. Scribing can only use for PP with a thickness of 0.8…2 mm.

For boards with a thickness of 2 mm or more, it is advisable to use milling. If it is necessary to use drilling, it is necessary to coordinate with the head of the PP tracing department. The minimum value is specified for diameter D. The preferred value is 0.4 mm. The minimum width of the jumper should be 1.5 mm, the maximum – 4 mm.

The jumpers must position in such a way that they can then easily remove with wire cutters. It is forbidden to place jumpers under the components of the housing.

The maximum distance between the jumpers on the board should be 75 mm.

A free zone in the central part with a width of 5 mm is required for the workpieces for the possibility of soldering in a reflow furnace with central support. The free zone is a section of the PP free of SMD components. If possible, avoid placing conductors and contact pads in the free zone that are not covered with a mask.

If necessary, an additional technological field can be introduced for the implementation of the free zone

Design of printed circuit boards with microcircuits in BGA cases

The shape of the contact pad for the ball pin is a circle with a diameter from 2/3 to 3/4 of the diameter of the ball pin of the component, but not less than 0.20 mm.

The contact pads for the BGA terminals should completely open from the mask (nonsolder mask defined lands). At the same time, the value of the opening does determine by the step of the pins of the chip as follows–With a step of more than 0.8 mm, the opening should be 100 microns; At a step of 0.8 mm or less, the opening should be 50 microns.

When connecting the contact pad and the hole, the hole and the conductor suitable to it should be completely covered with a mask, while the width of the conductor should not exceed 75% of the diameter of the pad. Otherwise, the solder may leak into the hole

Summary

The modern level of electronics development imposes increased requirements both to manufacturers of printed circuit boards and modules and to developers.
Often, in modern developments, these two sides face a contradiction between the need to use one or another type of components, materials, technologies, and the ability of manufacturers to do it with minimal costs and with high quality.
The task of the PCB designer is to find the “golden mean” that would satisfy all parties involved in the project implementation process from its development to the production of the PCB and assembly.

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