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Rush PCB Inc is the #1 pcb manufacturer in USA. Our services include Printed Circuit Board manufacturing , pcb fabrication, pcb design and pcb assembly services.

Fiberglass Fabric Styles Used in Laminates

Written by Rush PCB Inc on . Posted in PCB, PCB Fabrication

Eminent PCB fabricators such as Rush PCB Inc. use different types of prepregs and laminates for their PCBs. The manufacturers of these materials offer a number of fiberglass fabric styles. They base their selection on the thickness that the finished laminate or printed circuit board will take. It also depends on the amount of resin that will be present for filling and bonding. They make the specific choice depending not only on building up the thickness, but also on secondary properties such as cost, dimensional stability, CTE control, dielectric constant, and stiffness.

Construction of Fiberglass Boards

Manufacturers begin with fiberglass fabric on a warp beam that contains several thousands of individual strands of yarn rolled over a master beam. These yarns constitute the machine direction the fabric will take, which is also called the warp direction. They then slash the warp yarns, or run them through a solution of lubricants or sizing agents and this protects them from damage during the weaving process.

The actual weaving process begins by mounting the warp beam on the back of a loom, with the fill yarns being inserted as the warp yarns pass through from the back of the loom to its front. Earlier, there were the Draper looms, where a wooden shuttle with the fill yarn would proceed back and forth from one side of the loom to another to insert the fill yarns, while the alternating warp yarns moved up and down in a mechanical frame as it created the traditional plain weave, resulting in a woven or drapered edge.

The newer looms operate on air or water jet, where the jet carries the fill yarns across the loom. Cutting off the fill yarns individually leaves a fringed edge. In the modern looms, a single warp beam can contain several thousand meters of warp yarns that represent as much as a single loom can weave in a week.

Conductor Surface Roughness

After the weaving is over, the electrical grade glasses need to be scoured with an aggressive water rinse, which removes excess sizing from the warp yarns. To remove the balance, they are then heated in an oven at elevated temperatures for a long period. The weaver then treats the fabric with finishing agents such as organosilane that provides a surface that resin systems can wet and bond.

Glass intended for polyimide manufacturing requires application of a high temperature finish such as amino-silanes. This makes the bonds tough enough to withstand the use conditions the polyimide will go through.

Although the finish on the fiberglass fabric is only a very tiny amount of material, it vitally affects the way resin will wet the surface during the prepregging process. Poor scouring, heat cleaning, or inadequate silane treatment can leave the prepreg with repellent streaks or spots that often show up when heated. Glass weavers offer a variety of finishes, and the correct choice of finish for each resin determines its performance critically.

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Green Epoxy

Several epoxy resin systems are green in color. This started with fiberglass finishes that had chromium chemistry as their base with the trade name of Volan, which produced a green finish. Now, very few manufacturers use Volan, and use organosilanes instead. However, the complaints from end users about the change in appearance of their product have forced the suppliers to start dying their FR-4 products green to make them look same as before. That explains the reason for green being so common a color for epoxy printed circuit boards.

Smooth resin rich surfaces offer a better fill for internal etched copper patterns. This is often a result of lightweight fabrics with high resin content. Using heavier fabrics result in lower cost, while offering enhanced dimensional stability and permit building up greater thickness at lower cost per mil. However, the use of heavier fabrics usually affects drilling characteristics and surface smoothness. Although thicker and heavier fabrics result in low-cost rigid laminates, they can deflect small drill bits causing them to break.

Warp, Fill, and Direction of Weave

For a woven fabric, the term warp indicates the direction of the length of the roll, while fill indicates the direction of the yarns that fill in from side to side in the weaving process. In commonly used fabrics, the tensions and the number of yarns are not evenly balanced and that has a varying effect on stability and subsequently on registration. For a PCB fabricator, it is necessary to know the direction of warp and fill so they can orient them similarly each time and adjust the processing and compensate for predictable effects.

For instance, for a laminate measuring 36 x 48 inches, the warp is normally parallel to the longer dimension. Unless the customer makes a special request, or it is necessary to cut otherwise, the warp usually follows the longest dimension of a piece of cut panelized or prepreg laminate. When warp is not along the longest dimension, or for square panels, the fabricator marks the warp direction clearly by an arrow on the package or on the material.

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Weave Distortion

Under normal handling conditions, warp yarns are under tension and remain straight, while fill yarns should remain at right angles to them. However, for some reason, if some of the fill yarns in the fabric move away from their 90° position, the laminate or multilayer may develop a ripple or twist. For the fabricator, it is very important to have raw fabric with undistorted yarns. However, even for fabric with undistorted yarns, warpage can still occur unless the laminator takes care to align the yarns in one sheet of prepreg relative to another.

Summary

It is not easy to specify or measure the dielectric constant of the laminate, as it depends not only on its intrinsic properties, but also on the method of testing, conditioning of the sample before and during the test, and the test frequency. Moreover, dielectric constant tends to vary with temperature.

At Rush PCB Inc., we determine the characteristic impedance of a PCB based on the laminate thickness, its dielectric constant, and the height and width of the etched line height. Impedance matching and control are critical to linked functional modules when dealing with high-speed devices and designs.

Factors Affecting the Longevity of Copper Bond

Written by Rush PCB Inc on . Posted in PCB

Fabricators of printed circuit boards (PCBs), such as Rush PCB Inc., know that throughout the working life of PCBs, the bonding between the copper traces and the laminate is very important. Copper pads or traces detaching and lifting off the board may be damaged easily, and cause the board to fail. In applications where the device is likely to fall, such as in mobile phones, the requirement is the board has to resist a one-meter drop test. For low peel values, the G-shock of such a drop can cause devices to pop off the board along with the pads.

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Mechanism of Copper Adhesion

A combination of chemical and physical bonding allows copper to adhere to the laminate material. The natural tooth structure of ED copper foil formed during electro-deposition and its modular treatment gives it a locking mechanism, which the resin uses to coat and encapsulate. The bond is enhanced if the copper foil has been chemically treated. Fabricators treat foils with proprietary silane or other treatments, as this chemically enhances bonding to a variety of resins. However, all copper finishes do not work equally well with all resins. For achieving good bonds, it is critical to optimize the lamination process and select the proper foil finish for each resin system.

Resistance to Chemicals and Heat

Laminate materials such as Teflon and Polyimide can resist varying degrees of chemicals and heat. They usually have excellent tolerance to attack from various etching and plating chemicals. They can also withstand temperatures as high as 260°C without damage for prolonged periods. However, the bond between the laminate and the copper foil that will ultimately be the circuitry on the finished PCB is somewhat more delicate. Oxidation under the foil and shear from thermo-mechanically induced forces are the major causes of failure of the copper bond.

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Mechanism of Failure

Taking the path of least resistance, oxygen diffusing under the foil at the base of an etch line can start to further oxidize the copper interface. This typically happens when the PCB is exposed to high temperatures. Eventually the copper surface at the interface largely turns into a very weak black cupric oxide. This weakens the bond and it fails when stressed mechanically of thermally. Examining a peeled copper trace under a microscope usually shows a reduction of the bonded width as a result of prolonged thermal exposure.

It is not easy to predict the temperature levels and duration that causes the oxidization. However, start of oxidation is usually prevented or delayed with the use of conformal coatings that act as barriers to oxygen diffusion, or when the PCB assembly is used under inert gas blanketing, rather than being exposed to air. Long exposure to elevated temperatures also extends the service life of boards greatly, such as during burn-in for boards with exposed pads-only on their surface.

As the temperatures cross Tg, the glass transition temperature, epoxies or thermosetting polymers may soften and thereby lose their bonding capability, causing copper pads to lift off the epoxy board easily during field solder repair or rework. However, this problem is not prevalent among polyimides, as they rarely exceed their Tg temperature during soldering. Additionally, the use of newer, lead-free solder systems aggravates the issue, and it becomes critical when devices on the board need to be removed and reattached.

Mechanisms to Enhance Bonding

At Rush PCB Inc., we use several copper treatments to reduce the effects of secondary oxidation attacks, and to enhance the copper bonding. In addition to the normal structure of the grain, we treat the copper foil to enhance the growth of copper nodules that act as bonding teeth. Light oxidation and a subsequent treatment such as a deposition of nickel, zinc, or brass, followed by a treatment of chromate conversion or other oxidation inhibitors are usually successful in reducing subsequent oxidation attacks.

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What You Should Know About Flex and Rigid Flex Circuit Board Manufacturing

Written by Rush PCB Inc on . Posted in PCB, PCB Manufacturing

Rush PCB Inc. makes different types of flex and rigid flex PCBs starting from single sided to double sided, and multi-layered. These boards are special in the sense they can be bent either once, such as when they are being installed in a miniature camera, or bent multiple times such as when attached to the head of an inkjet printer.

Flex and rigid flex circuit board manufacturing has a fundamental difference from conventional PCBs in their use of basic material—this does not contain any glass reinforcement. Flexible circuits usually make use of various grades of Polyimide as their core or base dielectric. The Polyimide serves to provide both flexibility and mechanical integrity. On this Polyimide dielectric, a copper clad is bonded with a layer of adhesive called bondply (as against the conventional prepreg). After etching, this copper layer of traces is covered with a coverlay (similar to the conventional soldermask).

Flex and Rigid Flex Circuit Board Manufacturing

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image; ncabgroup

Circuit board manufacturers of flex and rigid flex start with the Polyimide base dielectric layer. Rush PCB Inc. uses Polyimides of various thicknesses from 12 µm to 125 µm, depending on the requirement. On this, they bond a rolled-annealed copper foil that can vary in thickness from 9 µm to 70 µm. Manufacturers use thermo-bond adhesives to bond the copper foil to the Polyimide base. These adhesives may include acrylic, phenolic butyral, and modified epoxy.

Panel Preparation

Fabricators first check the copper surface for imperfections such as dents and pits to ensure it meets the procurement standards. Usually, the manufacturer then bonds the copper sheets on to one or both sides of the Polyimide base using the adhesive in between. Application of heat and pressure in a lamination process makes the adhesive flow and bonds the copper foil to the Polyimide base.

As all the materials used are flexible in nature, handling techniques are crucial for the successful processing of flex circuits. Manufacturers modify their techniques and equipment adequately to support the thin laminates.

The fabricator then cuts down the full size sheets to usable panel sizes using shearing machines. Companies also use a standard fabrication panel size that fits optimally on all their production machines. Further processing of the panels begins with a baking process, which uses controlled heating for a defined period and a slow cooling process to remove moisture and helps to balance any internal stresses evenly.

Drilling

This stage begins by placing the panel between phenolic fiberboards to support it. Two or more registration holes along the edges of the panel serve to line up all the various features of the panel, including the drilled holes and the conductive pattern. The registration holes will also help in all stages of fabrication to align the panel on all the production machines at each step of the manufacturing process.

The registration holes help to align the panel on a numerically controlled drilling machine. The NC drill file supplied by the customer serves to program the drilling machine and to control the several heads with their corresponding tooling pins. The computer in the drilling machine moves the table to the proper location in both the x and y directions. The drilling head then picks the appropriate drill bit and creates holes in their programmed locations.

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Through-Hole Plating

At this stage of circuit board manufacturing, the fabricator chemically deposits a very thin layer of electroless copper over the entire surface of the panel including on the inside of the holes. With the holes thus metalized, an electrical connection forms from one side of the flexible circuit to the other.

Imaging and Developing

The fabricator now prepares the panel for application of the conductive paste or circuit imaging. The surface preparation may consist of chemical dipping into an acid bath, followed by chemical cleaning, micro-etching, and application of an anti-tarnish agent.

Application of the image is usually accomplished by using a dry film, a screen printing process, or using a liquid photoimagable resist. The dry resist is laminated on to the panel using heat rollers. A sheet of film containing a negative image of the desired trace pattern is then placed on the coated panel.

Exposing the panel and the film to an ultraviolet light source hardens and fixes the specific areas of the resist under the transparent parts of the film, leaving the areas where the UV light does not penetrate as relatively soft and unexposed. The next stage is the developing process, where the unexposed soft resist is washed away, revealing the unwanted copper.

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The Etching Process

Fabricators use a chemical solution to dissolve the unwanted copper from the surface of the panel, leaving only the required copper circuitry under the photoresist. Once the hardened photoresist has been chemically stripped away, the desired copper pattern remains on the Polyimide surface.

An inspection process, involving visual and automatic optical inspection equipment, usually follows this stage. Sometimes, electrical test processes are also used.

Multilayer Rigid Flex Boards

Each additional layer is drilled and bonded on the base panel followed by the above process of through-hole plating, imaging, development, and etching. The fabricator bonds the rigid parts of the board also in the same manner.

Coverlay Application

At the final stage of the rigid flex circuit board manufacturing, the fabricator uses a coverlay to mask off all the copper areas of the flexible circuit that will not be soldered. This is an adhesive film prepared by drilling holes in the appropriate places, and the fabricator laminates this layer onto the flexible PCB.