On Mon, 13 May 2013 18:36:03 -0000, you wrote:
>cheers for the advice I will try another test strip but go over longer time.
>The uv box is a converted scanner with four UV 9W tubes from a nail curing unit
>The boards are positive photo from Maplins
>it is clear acetate lazer printer. I print 2 images to double up the thickness
As a complete guess on the exposure, look for a good exposure to be 3
to 4 minutes. This is based on my own experiences, different
photoresist, more UV light (perhaps).
I do think that 15 seconds is way too small.
Positive photoresist is a polymer (long chain molecules). It is
damaged (read unlinked, read broken apart) by UV light. This happens
from the top down to the board surface. The mask prevents that from
happening.
Under UV light, the top down starts to change into a form that can be
dissolved. Note that the stripper is just a more concentrated
solution, so we're dealing with degrees, not absolutes.
Underexpose, and with too strong a developer, you tend to strip the
board. That very thin layer of photoresist may not work to keep the
etchant away.
You may want to check the strength of your developer. IIRC, it took
very little time (15 to 30 seconds) to "develop" photoresist.
Harvey
>Dave
>
>--- In Homebrew_PCBs@yahoogroups.com, "smilingcat90254" <smilingcat@...> wrote:
>>
>> One thing not mentioned so far is to cure the image after exposure-development.
>>
>> After the image is developed on the board, some recommend to cure the remaining resist by exposing the image to harden the image. Some resist may not be compatible with this step so check first.
>>
>> Another issue is resist adhesion. Lots of discussion on this too. I always wear rubber glove to keep the board free of oil from my hand and to keep chemicals off my hand. Some even have recommended to mildly pre-etch the blank then apply the resist. And so on...
>>
>> If you want to be accurate with exposure, you might want to use Stouffer's 21 step exposure guide. do a google search if you want one.
>>
>