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At UniVenture, we are asked a multitude of questions by both our customers. 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.
Q: What is a microvia hole?
According to the IPC-2226 definition, it is a hole with a diameter of up to 0.15mm. It is usually a blind hole drilled with a laser.
Q: What is meant by a blind via hole?
A: It is a hole that runs from an outer layer, but not through the entire PCB. These holes can be drilled mechanically or using laser technology.
Q: 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.
Q: What are the different types of HDI features?
The graphic below shows the different structures available for mass production and prototype.
Q: What is the minimum pad size contra hole on the outer/inner layer?
A: 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
For tighter constructions, please ask one of NCAB’s technicians for advice.
Q: What is the “aspect ratio”?
A: 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.
Q: When I need thicker tracks than standard, which track widths can I use?
A: 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).
Q: What type of via hole plugging is recommended?
A: 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. 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).
Q: 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 width and the general complexity of the PCB. 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).
Q: Do I have to use an FR4 material with a high 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.
Q: 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.
Q: 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.
Q: 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.
Q: Are RoHS compatible PCBs also halogen-free?
A: No, not necessarily. The RoHS directive prohibits two bromide flame retardants, PBB (polybrominated biphenyls) and PBDE (polybrominated diphenyl ethers). What is normally used in PCBs is a bromide flame retardant called TBBP-A (tetrabromo-bisphenol A).
Q: 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.
Q: What are the rules regarding flame retardants, was there a national ban against TBBP-A which dominates in electronics?
A: No, the investigation found that it is, for practical reasons -not possible to ban.
Q: What is the difference between flame retardant added in reactive respectively additive form?
Reactive flame retardant is chemically bound to the epoxy and will not dissolve at waste deposit.
Q: How many reflow cycles can FR4 materials withstand?
It is hard to give a precise answer, but we have made tests both on Dicy and noisy-material with up to 22 reflows, four of these with a peak temperature of 270C°. The stress after 22 reflows is considerable but all connections stick together. Our recommendation is to choose a high Tg material where there are more than 6 layers and thicker than 1.6mm. The picture to the right shows a nondictatorial and the left picture a Dicymaterial.
Q: What do the component acronyms BGA, SMD, QFP stand for?
A: SMD means surface mount device and can cover such components as resistors, capacitors and transistors. Such components do not have legs for through-hole assembly and therefore can be very small in size – down to 0.41 x 0.20mm (foot print).
QFP stands for quad plat package and this is a surface mount integrated circuit package with leads on all four sides.
The image below shows a basic QFP and the associated footprint on the PCB.
BGA stands for ball grid array, and this is an integrated circuit package with one face covered (or partly covered) with pins in a grid pattern. These pins conduct electrical signals from the integrated
circuit to the printed circuit board (PCB) on which it is placed. In a BGA, the pins are replaced by balls of solder stuck to the bottom of the package.