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‘Understanding the Importance of Precision’
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FR4... Well ok, but what is it?
FR 4
Well ok……….but what is it……….?
What dose it all mean?
Well if only I knew... this article could be up their with Paul McKenna’s “Change Your Life in Seven Days” in the Times bestseller list.
FR4 describes the laminates most frequently used in the manufacture of all but the most massively mass produced electronics today.
FR indicates the laminate is Flame Retardant (to UL94V-0).
4 (long story) indicates that the core of the laminate consists of woven glass reinforced epoxy resin.
Oh oh is it really flame retardant daddy?
That doesn’t look very flame retardant now does it?
The PCB on the right has been torched (well I had to have a little fun writing this now, didn’t I?), although it has seen better days, almost immediately upon removal of the flame from the blow torch the circuit ceased to flame, in addition (and perhaps most importantly) it did not drip any flaming material while being burnt or afterward.
In actual fact I believe UL test laminates by subjecting them to similar treatment with a piece of cotton wool beneath the laminate being tested, if the cotton wool dose not catch fire then approval is granted (we all pay them for this by the way).
How Is It Made…
Well actually it’s manufactured in much the same way that we manufacture multi-layer PCB’s, during the recent shake up of laminate suppliers many PCB shops found themselves making non standard laminate thicknesses from pre-pregs and copper foil due to excessive lead times for delivery of laminates from our suppliers.
FR4 is made from sheets of pre-preg, which is constructed from fibre glass cloth which has been impregnated with the epoxy resin. A standard 1.6mm circuit board is made up from 8 layers of pre-preg and the outer copper layers. The pre-preg and outer copper foil layer are stacked up together and placed into a large press which bonds all the layers into the final laminate.
The copper foil is made on a slowly rotating stainless steel drum submerged in a copper plating solution. As the drum turns copper is electroplated onto the drum, which is then stripped off as the plated section leaves the solution. It is worth noting that this type of copper foil is sometimes referred to as ED copper (electrodeposited), the copper used for flexi circuit materials is often RE (rolled enamelled), RE copper is less likely to fracture when flexed but more expensive to fabricate.
The glass cloth is literally made by weaving fine strands of fibre glass yarn together (little chap, name’s Rumplestiltskin). The yarn is in turn made from small threads of glass twisted together. Its the toughness of the glass in the cloth that give the laminate it's strength, and also the ability to take the edge off a steel drill bit in seconds (the drills used to make holes in PCB’s are generally tungsten carbide).
The epoxy resin is normally made from ethylene chlorohydrin [epichlorohydrin] and bisphenol-A. Once made the resin almost indestructible apart from high temperatures or strong oxidising solutions.
Here endeth the lesson.
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