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.
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.
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.
Which PCB surface is best for lead-free soldering?
There is no “best PCB 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 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.
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.
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 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.
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℃
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 a high Tg for lead-free soldering?
No, it´s not necessary to use a FR4-material. 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 value. Note that even with “leaded” soldering the Tg (Tg = glass transition temperature) 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.