Pack and unpack (PAU) of through-hole PCB
This process is used to prepare through-hole arrays. Unlike other double exposure processes, this process does not directly provide additional graphic resolution. The graphic resolution is only obtained during the first exposure, and the second exposure is just to select the required through a hole in the dense array.
Main difficulties of the pack and unpack (PAU) of through-hole PCB
The main difficulty in the PCB manufacturing service of this kind is how to cure the first layer of photoresist, that is, it is necessary to avoid damage to the first layer of photoresist during spin-coating of the second layer of photoresist and the influence on the first layer of photoresist during the second development.
Two solutions are proposed for this.
First, the two photoresists use different solvents, and the first layer of photoresist is insoluble in the solvent of the second layer of photoresist. The first layer of photoresist is soluble in ethanol but insoluble in PGMEA.
Second, the UV curing process is used to fix the first layer of the photoresist pattern. The curing process makes the first layer of photoresist very firm and prevents it from being dissolved by the second layer of photoresist solvent.
Grouping of through-hole PCB
For professional circuit board manufacturing companies, the PCB plated through-hole is traditionally divided into two groups: plated (supported) holes and non-plated (unsupported) holes. "Supported" refers to the plating on the hole wall. Non-plated or unsupported holes may or may not have bonding pads, such as mounting holes and non-porous wall plating. This is a manufacturing term, but for design, holes should be divided into two types: welded and unwelded.
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