Phonetic spelling and dialect often go hand-in-hand. It’s a way to capture the genuine sound of language but it can be challenging for your reader.
When to Write in Dialect
Let’s say your character is a Scottish immigrant. In order to establish 
his unique sound, phonetic spelling is your best bet. This will not only give him a voice distinct from his fellow characters, it will introduce challenges of interpretation. Down becomes doon, no becomes nae. Some characters will be unable to understand a heavily accented Scot, while others will misinterpret his words completely.
Why Phonetic Spelling?
You want your reader to ‘hear’ the words as your Scot pronounces them. In addition to the accent, Scots have a different vocabulary. This can lead to all kinds of fun for the writer. You can complicate things further by having two characters whose dialects clash. I once wrote a story of a Scottish fishing guide who offered an Australian a shiel with perks. The Australian thinks the guide is offering an illicit union (Sheila is an Australian term for a young woman) and they almost come to blows until a third character, Duff translates:
“Look here, man.” Duff put his heavy hand on Geoff’s shoulder. He struggled to keep his good cheer under control. “In these parts a shiel is a hut along the riverbank. To escape the weather. A perk is a fishing rod. So you see, a shiel with perks is—” The laugh that burst from Duff was as powerful as a river bursting its banks.
Use Phonetic Spelling Sparingly
Phonetic spelling allows your reader to experience the dialect as if in the 
room with your character. The challenge is to keep phonetic spelling to a minimum. Otherwise, your poor reader will feel like they’re reading a foreign language, spending more time interpreting the sounds than understanding the meaning. Choose enough words to give the flavor of the dialect without overwhelming your reader.
Below are a few examples from classic works of literature.
To Kill a Mockingbird: “Nothin’s wrong with him. Naw, Jem, I think there’s just one kind of folks.”
Huck Finn: “It didn’t take me long to make up my mind that these liars warn’t no kings nor dukes at all, but just low-down humbugs and frauds.
When done in moderation phonetic spelling can be a great tool to capture your character’s unique voice.
