FAQ about printed circuit boards

At NCAB Group, we are asked a multitude of questions by both our customers and our own team. Below we have listed some of the most common questions we receive, and those aspects which are common discussion topics. Hopefully this list will help, be a source of reference or perhaps the trigger for a more in depth discussion.

If you cannot find the answer you are looking for, or would like more details, then please contact us and we would be more than happy to assist.

Questions

HDI

What is a microvia hole?

A microvia hole

According to the new definition within IPC-T-50M a microvia is a blind structure with a maximum aspect ratio of 1:1, terminating on a target land with a total depth of no more than 0.25mm measured from the structure’s capture land foil to the target land.

What is meant by a blind via hole?

It is a hole that runs from an outer layer to the inner layer, but not through the entire PCB. These holes can be drilled mechanically or using laser technology.The image shows a laser drilled blind via.

A microvia hole

What is meant by a buried via hole?

This is a hole that runs between one or more inner layers. They are normally mechanically drilled.

buried via hole

What is a HDI PCB?

IPC-2226 defines HDI as a printed circuit board with a higher wiring density per unit area than conventional printed circuit boards (PCB). They have finer lines and spaces ≤ 100 µm / 0.10mm, smaller vias (<150 µm) and capture pads <400 µm / 0.40mm, and higher connection pad density (>20 pads/cm2) than employed in conventional PCB technology.

A HDI PCB

Are there different types of HDI features?

The graphic below shows the main structures – type I, type II and type III as defined in IPC-2226.

Type I. Defines a single microvia layer on either one or both sides of core. Uses both plated microvia and PTH for interconnection, employing blind, but not buried vias.

Type 1 HDI features

Type II. Defines a single microvia layer on either one or both sides of core. Uses both plated microvia and PTH for interconnection. Employs blind and buried vias.

Type 2 HDI features

Type III. Defines at least two layers of microvia on either one or both sides of core. Uses both plated microvia and PTH for interconnection. Employs blind and buried vias.

Type 3 HDI features

Construction terminology to define the degree of HDI construction:

  • 1+n+1 = single layer of microvia (as per the type I and type II examples above)
  • 2+n+2 = 2 layers of microvia (as per the type III example above)
  • 3+n+3 = 3 layers of microvia

FLEX/RIGID-FLEX

What are the available structures for flex and rigid flex PCBs?

There are numerous, different structures available. The more common ones are defined below:

Single sided flex (IPC-60103 type 1) Coverlay (polyimide + adhesive) bonded onto an adhesiveless single sided FPC core. With or without stiffeners.

Single sided flex structure

Double sided flex (IPC-6013 type 2) Coverlay bonded onto both sided of an adhesiveless double-sided FPC core (two conductive layers) with plated through holes. With or without stiffeners.

Double sided flex structure

Multilayer flex (IPC-6013 type 3) Coverlay bonded on both sides of an adhesiveless construction containing three or more conductive layers with plated through holes. With or without stiffener. Capability is 4L.

Multilayer flex structure

Traditional rigid flex construction (IPC-6013 type 4) Multilayer rigid and flexible circuit combination containing three or more layers with plated through holes. Capability is 22L with 10L flex layers.

Traditional rigid flex construction

Asymmetrical rigid flex construction, where the FPC is situated on the outer layer of the rigid construction. Containing three or more layers with plated through holes.

Asymmetrical rigid flex construction

Multilayer rigid flex construction with buried / blind via (microvia) as part of the rigid construction. 2 layers of microvia are achievable. Construction may also include two rigid structures as part of a homogeneous build. Capability is 2+n+2 HDI structure

Multilayer rigid flex construction

Book-binder and air-gap build – a complex build. With spaces between layers of flex to allow for improved flexing of the FPC. Flex layers may be of different lengths on the book-binder constructions to minimize compression of inner flex layers within bend radius.

Book-binder and air-gap build

How do you calculate correct length of flex part of a rigid flex structure to allow for minimum bend radius?

Detailed recommendations can be found in IPC-2223, sections 5.2.3.3 for both static and dynamic bends and should be used for final design verification. However, some basic guidelines based upon the thickness of the flex material can be seen below:

Illustration of a correct length of flex part of a rigid flex structure
FLEX TYPEMINIMUM BEND RADIUS
Single sided flex3 – 6 x circuit thickness
Double sided flex7 – 10 x circuit thickness
Multilayer flex10 – 15 x circuit thickness
Dynamic application20 – 40 x circuit thickness

What is the difference between semi-flex and rigid flex?

Semi flex is effectively a ‘standard’ multilayer PCB, built using specific types of FR4 that has been depth to a specific tolerance so that the resulting thinner area provides a flexible / bending section of the traditionally rigid FR4.
Semi flex is suitable for static applications (flex to install), or with very limited number of bends.

Illustration of the difference between semi-flex and rigid-flex

What is the difference between adhesive and adhesiveless flex systems?

Builds that use adhesive systems rely on adhesive to bond the copper to the flex core. As such this adhesive layer along with full coverlay, penetrates into the rigid construction and directly into the hole. The greater expansion coefficient of the adhesive increases the risk of through hole problems, such as cracks and via hole failures as a result of the expansion, while having this material within the hole itself can lead to hole wall copper formation issues resulting from poor desmear of adhesive.

Illustration of the difference between adhesive and adhesiveless flex systems.

The solution to improving via / through hole reliability was to move towards an adhesiveless copper clad system (copper bonded directly to the polyimide) and limit the overlap of the coverlay into the rigid part of the board. This ensures a much more reliable through hole structure without adhesive penetrating into the hole. This is by far the most common approach to rigid flex.

DESIGN RULES

What is the minimum pad size hole on the outer/inner layer?

This varies from manufacturer to manufacturer, but in general you can say that the majority of manufacturers can produce them as follows:
A = 0.15 mm
B = 0.20 mm
C = 0.30 mm

Illustrating pad size hole
For tighter constructions, please ask one of NCAB’s technicians for advice.

When I need thicker tracks than standard, which track widths can I use?

Track widths of pcb

In general, the thicker the copper base, the wider the track should be. One rule of thumb is that with a 18 µm copper base the track should not be narrower than 0.1 mm (4 mil) and with a 105 µm copper base the track should not be narrower than 0.25 mm (10 mil).

How much finished copper can be expected?

There is a misperception that copper weights offer certain exact thicknesses and that these do not reduce during the production of a PCB. For example 1 oz. = 35um or ½ oz. = 18um.
However IPC-6012 detailed the acceptable minimum thickness of both copper foils and foils after plating based upon allowable tolerances of copper foils and reductions of plating copper during subsequant processing.
Below shows some the more standard copper weights and allowable finished thickness.

As such such it is critical to understand what you need and specify correctly – if not then you may under-specify or you may over-specifiy which can lead to excess costs being built into the design. For more information, please consult our technicians.

Inner layer foil thickness after processing
Base copper weightMinimum finished after processing
1/2 oz.11.4 um
1 oz.24.9um
2 oz.55.7um
External conductor thickness after plating and processing
Base copper weightMinimum finished after processing Class 2Minimum finished after processing Class 3
1/2 oz.33.4um38.4um
1 oz.47.9um52.9um
2 oz.78.7um83.7um

Should we remove or keep non-functional pads?

It’s clearly defined in IPC-2222A point 9.1.4 that the fabricator shall not remove these, but many factories ask they allowance to remove these to achieve a better yield in production. If they are not critical for the function, we recommend consider allowing their removal.

Illustration of removing or keeping non-functional pads.

TERMINOLOGY

What is the “aspect ratio”?

The relationship between the diameter of the hole and its length. When a manufacturer states that their production has an “aspect ratio” of 8:1 it means, for example, that the hole’s diameter is 0.20 mm in a 1.60 mm thick PCB.
For HDI structures, the aspect ratio for microvia is limited to 1:1, but 0.7-0.8:1 is preferable to easy the plating.

Illustration of aspect ratio

What is copper wrap?

Copper wrap is a continuous deposit of plated copper that is deposited within the barrel of the hole and extends onto the surface of the PCB (or the surface of the innerlayer core if part of a HDI structure) by a minimum of 25 um.

PCB with copper wrap

For class 2 demands the thickness of the surface deposit of the copper wrap is minimum 5um, but for class 3 demands this will vary dependant upon where this feature is situated within the build. Please consult our technicians for further information on class 3 demands.

What is controlled impedance?

Consider the PCB signal as it travels along a track controlling impedance means that we control the performance or speed of that signal at a point along a track. Related to resistance, capacitance and conductance of the track in question. The impedance is also measured in Ohms, it is different than resistance which is a DC characteristic. Impedance is an AC characteristic, meaning that it is related to frequency.

Is there more than one type of controlled impedance?

Yes, there is and these are explained below:

Differential impedance – Impedance of a pair of conductors with equal and opposite polarity signals – same amplitude in anti-phase with each other.

Odd mode impedance – Impedance of one side of a pair of conductors which both have equal and opposite polarity signals – same amplitude and anti-phase.

Even mode impedance – Impedance of one side of a pair of conductors which have equal signals – same amplitude and same polarity.

Common mode impedance – Impedance of a pair of conductors which have equal signs – same amplitude and same polarity.

What is meant by a thermal pad?

For SMD components, especially the smaller ones, use thermal relief pads to simulate the same thermal mass on both terminations to avoid tombstoning, twisted components or even in worst case broken components.

Close-up of a thermal pad
Close-up of a thermal pad

For hole mounted (HMD) components we use thermal relief pads on inner layer planes to help wetting in the hole barrel. Formulas for thermal relief pads can be found in IPC-2220 series.

Close-up of a thermal pad

Should approval for addition of teardrops be added in the procurement documentation?

According to new IPC-6012D section 3.4.2 it is allowed to add teardrops to the tracking when boards shall comply to classes 1 & 2. 

Illustration of adding of teardrops in the procurement documentation

If you do not want tear-drops, highlight that in the procurement documentation. But add enough annular ring.

What is back-drilling technology?

For high frequency applications we need to prevent signal loss and therefore when one layer is connected to another and the signal travels from one layer to another it must pass through a via hole that connects the layers. If the signal is to pass layer one to layer two in a 20 layer board, for example, then part of the via structure is considered as ‘excess’ and for this application it is best to remove the excess copper from this hole as it acts as an antenna and impacts the signal.

We use back-drilling (controlled depth in z-axis) to drill out the ‘excess’ copper in the hole in order to get better signal stability. Ideally the shorter the stub (‘excess’ copper) then the better the result. Back-drill size should be typically 0.2mm larger than corresponding via.

Illustration of counter bore and back drilling technology.
BACK DRILLING / COUNTER BORE (mm)
A: Depth tolerance+/-0.20+/-0.15+/-0.10
B: Min. remain thickness and tolerance0.5+/-0.250.35+/-0.10.3+/-0.1

What does “UL marking” mean?

Security is key in the electronics industry. It is vital that users can rely on the finished product when considering factors such as fire and electrical safety, which means that both the PCB and the materials they contain must measure up to the highest standards. To ensure that the boards do conform, it has become common practice to UL certify the constituent materials or the PCB itself.

To start with, what is UL? The letters UL stand for Underwriters Laboratories Why is UL recognition important? UL is a major authority and enjoys a good reputation in the PCB industry. The company is irreplaceable for testing fire and electrical safety. Customers from around the world – including well-known Chinese companies – demand factories that can achieve UL recognition. Having UL recognition is therefore very important for PCB factories to produce secure boards and be able to access the international market.

VIA HOLE

What type of via hole plugging is recommended?

The preferred type of plugging for standard product (not including capped via hole) is IPC 4761 type VI filled and covered, with target being complete fill. The image below shows type VI with liquid soldermask coverage.

PCB type VI with liquid soldermask coverage

Single sided plugging is not recommended (including type II tented and covered) due to concerns over entrapment of chemistry or likelihood of solderballs being present with HASL finishes (LF and SnPb).

What is a capped via hole?

A capped via is when plating is added over the via hole so that the surface is fully metalized with a minimum copper / cap plating thickness of 5um for class 2 demands, or 12um for class 3 demands.

This is reliant upon the via filling material being epoxy resin as opposed to soldermask, as the epoxy will minimize the risk of air bubbles or expansion of the fill during soldering operations.This can be categorized within IPC-4761 as type VII – filled and capped via holes. It is typically used for designs with via in pad or in BGA applications where high density features are required.

MATERIAL

Do I have to use an FR4 material with a high Tg (Tg = glass transition temperature) for lead-free soldering?

No, not necessarily. There are many factors to be taken into account, e.g. how many layers, the thickness of the PCB and also a good understanding of the assembly process (number of soldering cycles, time above 260 degrees, etc.). Some research has shown that a material with a “standard” Tg value has even performed better than some materials with a higher Tg value. Note that even with “leaded” soldering the Tg value is exceeded.
What is of most importance is how the material behaves at temperatures above the Tg value (post Tg) so knowing the temperature profiles the board will be subjected to will help you look evaluate the necessary performance characteristics.

What are the material characteristics to look for when selecting material?

 
The main ones that we would consider first include:

CTE
A measure of how much the material expands when heated. Critical in Z-axis – typically above Tg and the expansion is greater. If CTE insufficient for then failures can occur during assembly as the material expands rapidly above Tg.

Left image: Barrel crack/broken hole      Right image: Lifted land

Materials can have same Tg yet different CTE’s – lower CTE is better. Equally some materials can have higher Tg values, yet also have a higher (worse) CTE post Tg.

Tg / GLASS TRANSITION TEMPERATURE
The Tg value is the temperature at which the material changes from a reasonably stiff glass-like material to a more elastic and bendable plastic-like material. Important as above Tg, the materials properties will change.

Td / DECOMPOSITION TEMPERATURE
This is a measure of the degredation of the material. The analysis-method measures when 5 % of the material is lost by weight – the point as which reliability is compromised and delamination may occur.
Higher reliability PCB will require Td ≥ 340℃

Degradation of epoxy resin within FR4.

T260 / T288 / TIME TO DELAMINATION
This is the method to determine the time when the thickness of the PCB is irreversibly changed at a predefined temperature (260 or 288 in this case) – i.e. when the material expands to such a degree that it delaminates.

Do I have to use an FR4 material with the highest Td (Td = decomposition temperature) for lead-free soldering?

A greater Td value is preferable, especially if the board is technically complex and exposed to a number of remelting solderings, but this can lead to higher costs. Knowing your assembly process can help make the right choices.

What is the difference between “Dicy” and “nonDicy” as a hardening system in the FR4 epoxy?

Dicy (Dicyandiamine) is by far the most common hardening system for this epoxy; it normally gives a Td value of about 300–310°C while a “nonDicy”, i.e. a Phenolic Cured Epoxy has a Td value of about 330–350°C and can therefore better resist the higher temperature.

What does “CAF” mean?

CAF (Conductive Anodic Filament) means that there will be an electrochemical reaction between the copper anode and cathode, which may result in an internal short circuit in the material.

Which PCB surface is best for lead-free soldering?

There is no “best surface;” all surfaces have their pros and cons. Which one you should choose depends on many factors. Please consult our technicians or review the information on surface finishes within this section of the website.

What are the rules regarding flame retardants, was there a national ban against TBBP-A which dominates in electronics?

No, the investigation found that it is, for practical reasons, not possible to ban.

What is the difference between flame retardant added in reactive or additive form?

Reactive flame retardant is chemically bound to the epoxy and will not dissolve and migrate out of the product as part of the waste deposit.

How many reflow cycles can FR4 materials withstand?

It is hard to give a precise answer, but we have made tests with material with up to 22 reflows, four of these with a peak temperature of 270C°. The stress after 22 reflows is considerable and material can degrade, but all connections remained functional. Our recommendation is to choose a higher grade material where there are more than 6 layers and thicker than 1.6 mm.

FR4 test board following multiple reflows

RoHS

Do the RoHS or WEEE directives require the PCB to be marked?

No, but for practical reasons PCBs that have lead-free HASL should be clearly marked stating their RoHS compatibility due to the risk of confusion with leaded HASL.

Are RoHS compatible PCBs also halogen-free?

No, not necessarily. The RoHS directive prohibits two bromided flame retardants, PBB (polybromided biphenyls) and PBDE (polybromided diphenyl ethers). What is normally used in PCBs is a bromided flame retardant called TBBP-A (Tetrabromobisphenol A).

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