Am redesigning my sign-up form, some users feel bored with filling security field. Is really the Security verification code is user-friendly?
No it's not.
But you have alternatives to captchas that are a bit better, check this article Top 10 Really User Friendly Captchas. There are other options, but in that post, you may fined a few good ideas.
There was an extensive research regarding CAPTCHAs usability and effectively, CAPTCHAs including reCAPTCHA, logic questions, image recognition, and even friend recognition (take a look at this ridiculous stuff).
So what is the perfect CAPTCHA?
None. None of them meet all of the requirement for user experience, each of them have downside due users having visual, sound, logic or language problems.
No CAPTCHA is the perfect CAPTCHA.
Of course, for user experience, it's perfect. But how do you protect from spam? There's a lot of ways, such as moderation against spam, honeypot method, client-side detection, server-side spam detection, or social moderation (upvote and downvote).
In my opinion, for a small website, honeypot method is the easiest and most effective solution. Basically honeypot is an additional field that is hidden to users but visible for spam bots. How does it work? Spam bots only interact with raw HTML rather than rendering the source code and due to that, spam bots won't detect the hidden field and if the hidden field is filled, then you can assure it was done by a spam bot. You can hide the field for users with
display: none with CSS for an example. Read more about honeypot method here.
To sum it all, it's best not to use captcha and use the alternative method I metioned above. However if you think it's really necessary to add captcha, then if you have a small website, I'd suggest you to use one of those unpopular captcha that spambot programmers won't bother doing it for your site.
Read more about CAPTCHA research here.