When you ask a PCB for fabrication, you need to specify details on PCB materials, impedance controls,PCB surface finish, copper thickness and so on. Most of the details are so common that they are always assumed. However still it is better to know them.
PCB Material Selection reference
Printed circuit board is called for core component of electronics. Alternately, the acronym has also accounted for printed circuit boards, printed wiring boards and printed wiring cards, which are essentially the same thing. Due to the crucial role of these boards in everything from computers to calculators, PC board material selection should be undertaken with care and knowledge for electrical necessities of a given piece of equipment.
Before the development of the PCB, circuit board materials were mostly covered by nests of entangled, overlapping wires that could easily fail at certain junctures. They could also short circuit once age took hold and certain wires started to crack. As could be expected, the manual process that went into the wiring of these early boards was confusing and painstaking.
As an increasing variety of everyday electronic components began to rely on circuit boards, the race was on to develop simpler, more compact alternatives, and this led to the development of the material, PCB. With PCB materials, circuits can be routed between a host of different components. The metal that facilitates the transfer of current between the board and any attached components is known as solder, which also serves a dual purpose with its adhesive qualities
PCB Material Composition
The composition of a PCB generally consists of four layers, which are heat laminated together into a single layer. The material used in PCB includes the following layers from top to bottom:
• Silkscreen layer
• Soldermask layer
• Copper layer
• Substrate layer
The last of those layers, substrate, is made of fiberglass and is also known as FR4, with the FR letters standing for “fire retardant.” This substrate layer provides a solid foundation for PCBs, though the thickness can vary according to the uses of a given board.
A cheaper range of boards also exist on the market that don’t utilize the same aforementioned PCB materials, but instead consist of phenolics or epoxies. Due to the thermal sensitivity of these boards, they tend to lose their lamination easily. These cheaper boards are often easy to identify by the smell they give off when being soldered.
PCB second layer is copper, which is laminated onto the substrate with a mixture of heat and adhesive. The copper layer is thin, and on some boards there are two such layers – one above and one below the substrate. PCBs with only one layer of copper tend to be used for cheaper electronics devices.
The massively-used Copper Clad Laminate (CCL) can be classified into different categories according to different classification standards including reinforcing material, used resin adhesive, flammability, CCL performance. The brief classification of CCL is shown in the following Table 1.
|Classification Standard||Material Type|
|Reinforcing Material||Paper base type||PF Resin(XPC,XXXPC,FR-1,FR-2)|
|Glass fiber cloth base class||Epoxy resin (FR-4,FR-5)|
|Composite epoxy material (CEM)||CEM-1.CEM-3|
|laminate material base class||N/A|
|Special material Base Class||Bismaleimide Triazine (BT) or equivalent;
Nelco N – 4000 – 13 (SI);
Nelco N – 4000 – 13;
Getek / Megatron;Taconic RF35;
Rogers 4350;Rogers 3003/3006;
Hitachi MCL-BE-67G;Polyclad PCL-FR370HR
|CCL Performance||CCL with ordinary performance||N/A|
|CCL with low dielectric constant||N/A|
|CCL with high heat resistance||N/A|
|CCL with low coefficient of thermal expansion||FR4 (S1440_(low CTE))|
Copper Clad Laminate reference Table 1
Below is our normal and main suppliers, if you need other other materials, please contact us freely.
|Company name||Data Sheet||Website|
|ARLON Materials for Electronics Division||Alron 35n;45n;37n||www.arlon-med.com|
|CHANGZHOU Chaoshun Electronic Technique Co.Ltd||Aluminum;Iron-Based||www.csdztech.com|
|DOOSAN Electro-Materials Co., Ltd.||DS-1108 (S); DS-1202||www.dse.co.kr|
|KINGBOARD Chemical Holdings Ltd.||KB6165;KB6167||www.kingboard.com|
|ROGERS Corp., Advanced Circuit Material Division||Rogers4350B;TMM3||www.rogerscorp.com|
|SHENGYI SCI.Tech CO.,Ltd||S1440_(low CTE)||www.syst.com.cn|
|TACONIC Advanced Dielectric Division||RF-30 ;RF-35||www.taconic-add.com|
|NAN YA PLASTICS CORPORATION||CEM-3-92(CTI>600)||www.npc.com.tw|
Above the green soldermask is the silkscreen layer, which adds letters and numerical indicators that make a PCB readable to tech programmers. This, in turn, makes it easier for electronics assemblers to place each PCB in the proper place and in the right direction on each component. The silkscreen layer is usually white, though colors such as red, yellow, gray and black are also sometimes used.
When you design PCB silkscreen layer,we would suggest you to fix below 2 items:
1. the width of the silkscreen letters as 5mil is best option, if your width is less than 5mil, the finished PCB silkscreen will be fuzzy;if the width is much more than 5 mil, the Silkscreen cosmetics of the finished PCB would be not good.
2. Do not place on the Silkscreen letters on a SMT PAD which will solder component,if that, the silkscreen letters will prevent(affect) soldering process
PCB Layer Technical Terms
Along with an understanding of how the PCB is layered, it’s also important to know the following technical terms that accompanies the use of PCBs:
- Annular ring. The copper ring that surrounds the holes on a PCB.
When traces on a circuit board need to connect to another layer, they typically end up on a pad and then through a via on that pad go to connect to another layer. An annular ring is the area on the pad that surrounds the via.
The width of the annular ring is important design and manufacturing consideration. The best annular ring size is 0.05mm (min)
During manufacturing there are many conditions that can cause the drilled hole to not be dead-center (even though that is how it is on your CAD design).
For example, during the drilling process, the drill bit may wander slightly, or during lamination process some of the layers may have shifted slightly, or during imaging the registration is not 100% dead center and so on.
In fact, it is almost never the case that the manufactured hole is exactly dead center on the pad as designed in the CAD.
Thus, if the designer provides a wide annular ring area in the design, it is likely to ensure that in the manufactured printed circuit board we still retain good electrical connectivity between pad and the hole.
If the annular ring width is not enough, then during manufacturing, the hole could be almost touching the boundaries of the pad (called “Tangency”) or in worst case, even go outside the boundary of the pad (“breakout”).
As long as the manufactured hole is inside the pad and even touching the boundaries of the pad, that is generally an acceptable situation. On the other hand, for certain applications or situations a designer may need absolutely that there should be a minimum left over annular ring (typically 1 or 2 mils) after manufacturing.
How do you calculate Annular Ring Width?
An Annular Ring Width is the (diameter of the pad – diameter of the hole) / 2
Example 1: if diameter of the pad is 22 mils and diameter of hole is 10 mils then annular ring width = (22 – 10 )/2 = 6 mils.
Example 2: if we know that on a certain pad, the annular ring width is 8 mils, that means that Pad diameter is (8*2 =) 16 Mils more than the hole diameter.
A Description of the ANNULAR RING OPTIONS when running Better DFM:
When running Better DFM you can go to Advanced Options and choose what you want the DFM to check for in your annular rings.
If you are okay with accepting Tangency on pads in the manufactured board, then our DFM will look for a minimum of 5 Mils (0.005″) annular ring width in your design.
(Tangency is the default option, and the DFM will check for 5 Mils minimum annular ring width)
If you wish to have minimum 1 Mils annular ring on pads in the manufactured board, then our DFM will look for a minimum of 6 Mils (0.006″) annular ring width in your design.
If you wish to have minimum 2 Mils annular ring on pads in the manufactured board, then our DFM will look for a minimum of 7 Mils (0.007″) annular ring width in your design.
If you are okay with accepting Breakouts on pads in the manufactured board, then our DFM will practically not look for any minimum annular ring width in your design.
(NOTE: accepting Breakouts is typically NOT recommended!)ed and un-tented versions. Tented versions are covered with protective soldermask, while the untented vias are used for connector attachments.
- DRC. An acronym for Design Rule Check (DRC) is the area of electronic design automation that determines whether the physical layout of a particular chip layout satisfies a series of recommended parameters called design rules. Design rule checking is a major step during physical verification signoff on the design, which also involves LVS (layout versus schematic) check, XOR checks, ERC (electrical rule check) and antenna checks. For advanced processes some fabs also insist upon the use of more restricted rules to improve yield.
Essentially in PCB field, DRC is a practice whereby the design of a PCB is checked for its functionality. Details that are checked include the width of the traces and drill holes.
- Drill hit. Used to describe all holes on a PCB, whether correct or misplaced. In some cases, a hole might be slightly incorrect due to dull drilling equipment used during the production.
- Finger. Normally we called it as gold fingers. Gold Finger is gold-plated terminal of a card-edge connector, usually, fingers are done by flash gold (hard gold) and the gold thickness is requested to be from 3u“ to 50u“ because fingers are mainly used for plugging for many times.When PCBs will be repeatedly installed and removed electroplated gold is used for edge-connector contacts or as they are more commonly known: Gold fingers.
- Mouse bits holes. (Also called stamp hole) A section of the board that has been overly drilled to the point where it threatens the structural integrity of a PCB. It is used to connect the break-away tab with the main PCB.
- Pad. An area of exposed metal on a PCB, onto which a soldered piece is generally applied.
- PCB Panel. A large circuit board consisting of smaller boards, which are eventually separated for individual use. The reason for this practice is to eliminate the difficulty that handlers experience when it comes to dealing with smaller boards. Normally the panel designed as mouse-bite-holes connection or V-CUT connection
- Paste stencil. A metal stencil on a board, onto which paste is placed for soldering.
- PCB Plane. A larger section of exposed copper on a PCB, which is marked by borders but lacks a path.
- Plated through hole. A hole that goes straight through a PCB, usually for the purpose of connecting another component. The hole is plated and usually features an annular ring.
- PCB Slot. Any hole that isn’t circular. PCBs with slots are often high priced due to the production costs of creating odd-shaped holes on a circuit board. Slots are typically not plated. Normally there are two different slots: Plated Slot and Non plated slot
- Surface mount. A method whereby external components are mounted directly to the board without the use of through holes.
- Circuit Trace. An ongoing line of copper across a PCB.
- V-score. A place where the board has been partially cut. This can render a PCB vulnerable to snapping.
- Via holes. A hole through which signals travel between layers. Vias are seen in tented and un-tented versions. Tented versions are covered with protective soldermask, while the untented vias are used for connector attachments.
The number that precedes a layer refers to the exact number of conducting layers.The layers may be either routing layer or plane layer. The commonly used no. of layers are 1, 2, 4, 6, 8 and other even numbered layers. Odd number layer boards may be formed but they are not standard boards and doesn’t offer any savings. The PCB materials used for 3 or 4 layers would almost be same.
Routing layers contains tracks while plane layer are generally used to connect power or ground. Plane layers have plane of copper from which connections are drawn up or down wherever needed. Islands can be formed on plane layers and each island may have different signal. Suppose some sets of chips on one board use 3.3 V while other use 5 V then the power plane may be partitioned into two islands of 3.3 V and 5 V.
FR4 specifies PCB MATERIAL TYPE – FR4 is another name for “Glass reinforced epoxy laminated sheets”. It is a commonly used PCB material. The reasons for its ubiquity as a PCB material are its high dielectric strength, high mechanical strength, light weight, resistance to moisture and other properties conducive for PCB manufacturing. FR in FR4 stands for “fire retardant”, another feather in this PCB material’s cap.
1.6 mm thick specifies FINISHED BOARD THICKNESS – In multilayer PCB it is thickness after all layers have been pressed together and all the processes completed. 1.6mm is default thickness for 4 layer board. Go for higher thickness if you have heavy components on board to get better strength. Keep an eye on thickness even when you have high voltages on board.
35 microns copper finish – The thickness of copper put on board. Another popular way to express is in grams or Oz. (Ounces per square foot) of copper per unit area. X Oz would mean that X Ounces were spread on the Board material and it was distributed uniformly over an area of 1 square foot.
Go for higher copper thickness if the current density on board is greater than in average digital circuits. In normal boards 35 microns is good. Up till a current of 1A and copper thickness of 1 Oz, track width of 10 mils is safe.
No impedance Control – When signal transition time is too small or frequency is too high then tracks may behave as transmission lines. If unchecked, such tracks may cause significant power loss. For maximum power to be transferred to other end of track, length and geometry of the track should be carefully laid taking care of transmission line equations.
As a rule of thumb, if the board is of FR4 PCB material with copper tracks and signal rise time is 1 nanosecond then 2 inches is a safe distance. If the track length exceeds this and signal integrity is important then consider the transmission line effects also. There are lot of free programs and excel sheet available on net that calculate impedances for prevalent geometries. If impedance control has been mentioned to the fabricator then an extra test is done on the completed PCB to check the required impedances.
No Via Filling – Normally vias are not filled and you clearly see other side of the board through them. However, they can be filled for various reasons. One need arises when vias are to be protected from solutions, dust etc. Vias can also be filled with conducting materials to increase the current carrying capacity of via. One more reason is to level the board. Via filling is needed many a times on boards with BGA (Ball Grid array) components. If BGA pins make contact with one of the inner layer then the via can’t be placed just underneath the pin because then the solder may flow in the hole and get connected to some other layer. So Via filling is done so that solder doesn’t flow in the via and contacts are properly connected.
No gold tabs – At any place on the board when something makes and breaks contact repeatedly then that area wears out fast. Example is a rubber pushbutton which has contacts printed on the board. An example is the kind of buttons used in calculator. This wearing out also happens when daughter card slot are provided on main board. The contacts on this slot are rubbed when card is pushed in or out. In such cases it is wise to put gold layer on the surface which makes contact. This enhances life of the contact. A separate step is added in the PCB fabrication process if gold tabs are specified. Also remember gold is costly.
Green gloss (LPISM) soldermask – The first image that would come to your mind when you think of PCB is a green colour board with some shining metal contacts and some holes. The green colour is because of this soldermask. Now why is soldermask used? Solder mask prevents spilling of liquid solder. This spilling has become a serious problem in recent times. As the pitch of the IC pins has decreased, due to this spilling the pins get shorted. To feel the miracle of soldermask, try to solder a board which doesn’t have solder mask (mostly a homemade PCB won’t have) and the one which has solder mask. You will feel a huge difference. This experience made me enjoy soldering which I hated earlier.
LPISM stands for Liquid Photo imageable solder mask. It is a type of soldermask. There are soldermasks available in other colours as well. Prevalent among them are Red and blue.
Leaded HASL finish – HASL stands for Hot Air Solder Leveling. After soldermask has been put on PCB. The PCB is passed through molten solder. All the copper which is exposed, like pads etc. get covered with solder. This is useful in actual soldering of the components. During soldering of SMD chips on board it is only needed to heat the board till solder becomes molten again and place the component at the respective position. The components get soldered as the molten solder again solidifies. No need to hold soldering guns, wires and flux.
Leaded HASL means that lead is also present in the solder compound put on the board. Nowadays HASL compounds without lead are also available.
Line Width / Spacing : 6/6 mils – This means the minimum track width would be 6 mils and minimum spading between tracks, tracks and pads and all other spacings should be greater than or equal to 6 mils. In case you don’t know mils, It is the prevalent unit of distance used in PCB world. 1 mil = 1/1000th of an inch = .0254mm.
Take care of width and spacing when higher currents and voltages are expected on board.
Electrical test, routed – In multilayer PCB, It is not possible to do visual inspection of all the tracks for their correctness. So this test is done. Test probes are put at the end of tracks or on pads such that signals on it can be accessible. A small voltage is applied on one such probe. If that voltage is detected at the other end, it confirms that the tracks are proper. This step may not be necessary in 1 or 2 layer board but if you don’t want to compromise on quality, get this test done.
PCB Material-Isola 370HR-High Tg : Isola is a famous company for PCB materials and 370HR is one of their product’s name. This is an FR4 PCB material. This PCB material in addition to FR4 properties has additional advantages like superior thermal performance which helps in dissipating heat on board. The mechanical, chemical and moisture resistance properties all equal or exceed the performance of traditional FR-4 materials. “High Tg” stands for “high glass transition temperature”. This means that the PCB made of this material can sustain high temperatures. During HASL or soldering, PCBs have to sustain high temperatures and that’s where this parameter becomes important.
No Blind/Buried Vias – Blind Vias are those vias which join one of the outer layers (TOP or BOTTOM) with one of the inner layers. Since this via can be seen from only one side, it is called a blind via (This may not seem logical but just remember itJ). Buried vias are vias which connect one of the inner layers to another inner layer. This vias cannot be seen from any of the outer layer and hence is called buried vias. Via filling as we studied above is used many a times when blind/buried vias are used. It helps level the surface just above via so that pads/tracks can be placed over the vias without the solder penetrating in.
Blind via, buried via, via filling, gold tabs put additional cost on PCB manufacturing. Also, if you go down 6 mils in line/width spacing, the cost goes up. This is because the fabrication processes change a little and also the rejection rate is high. There are PCB fabricators which can go down till 3 mils. Now some may have gone below that also, but don’t try to touch those limits until you don’t have other options.
The parameters defined above are just some of the values to define a PCB Circuit Board.