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I have been making the PCB boards by the conventional process since last few years (pressing, etching, drilling, soldering).

I usually iron the photo paper to the PCB board for around 15-20 minutes but this time something unusual happened and the PCB board had this shape. The copper rose from the board while I was pressing and left it in this shape:

enter image description here

This is my first encounter, can anyone suggest why this happened? I pressed for only 6 minutes.

enter image description here

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    \$\begingroup\$ 15-20 minutes? I doubt I ever did more than 2 or 3 minutes, often only 20-30 seconds. Try some experiments of shorter duration, likely the results won't differ in terms of quality but avoid that damage. Also start by degreasing the copper (then avoid touching it) and maybe use a scouring pad to slightly roughen the surface for better adhesion. \$\endgroup\$ – Chris Stratton Aug 1 '19 at 13:46
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    \$\begingroup\$ Is this board larger than your previous attempts? Perhaps un-even heat distribution is the cause. Looks like the edges may have encountered different heat than the centre. Or inferior bonding of copper-to-substrate de-laminated the copper. \$\endgroup\$ – glen_geek Aug 1 '19 at 13:50
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    \$\begingroup\$ Hello Chris Stratton, i informed that previously i had a few boards pressed for durations upto 20 minutes and the size of the board was the same (6*6 inch). \$\endgroup\$ – Ahmer Quadri Aug 1 '19 at 13:55
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    \$\begingroup\$ Maybe you have a batch of low quality copper clad stock, try some from another source. This is not something that would typically be manufactured for hobby or prototype purposes, it is being made for industrial production and then re-sold likely as surplus (or even scrap), so it is not hard to imagine how low quality material could make it into a likely mysterious supply chain. \$\endgroup\$ – Chris Stratton Aug 1 '19 at 14:10
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    \$\begingroup\$ You could also try baking the board stock at a low heat for an extended period of time on the theory that they may have soaked up a lot of moisture that could be boiling off when heated. \$\endgroup\$ – Chris Stratton Aug 1 '19 at 14:14
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You are most likely using low quality laminate and too high transfer temp. As you see the paper has turned brown which is a sign of you temps being too hot. I make my PCBs by heating them for 5min on cotton setting of the iron. Try cutting the board and transferring each one independently, not all at once, helps with etching too. Apart from that I can see that you have very good transfer paper, the traces look great!

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You are seeing "blistering". I can't really see the material you're using but if it's something like FR1 paper base phenolic it won't take much heat for very long. A typical spec would be 10 seconds. 30 minutes is a very long time.

Most copper-clad laminates are made by large companies and are of rather high quality (in that they meet all the specifications) but the specifications and performance vary significantly between an epoxy-glass laminate like FR4 and the cheaper phenolics. In some cases you can buy FR1 and actually get material that meets FR2 specs because it's not cost-effective for the manufacturer to make both. Which might lead to a surprise if you get FR1 the next time and have not done your design work properly.

There are better grades within the epoxy-glass spectrum too, look for a higher "glass transition temperature" specification.

I have had even prototype boards made in FR2-like materials (actually the lighter color CEM materials) because the end product was going to be made of that (punched, rather than drilled for high volume) and we wanted to evaluate the design with as close as possible to the final materials. Anything heavy or exposed to end-user mechanical forces is a potential point of failure in such a single-sided board since you're depending on the feeble adhesive on a rather brittle material to provide mechanical strength.

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