Good Primer’s Characteristic
A melting temperature (Tm) in the range of 52℃ to 65℃
Absence of dimerization capability
Absence of significant hairpin formation (>3 bp)
Lack of secondary priming sites
Low specific binding at the 3' end (ie. lower GC content to avoid mispriming)
Uniqueness
There shall be one and only one target site in the template DNA where the primer binds, which means the primer sequence shall be unique in the template DNA.
There shall be no annealing site in possible contaminant sources, such as human, rat, mouse, etc. (BLAST search against corresponding genome)
Length
Primer length has effects on uniqueness and melting/annealing temperature. Roughly speaking, the longer the primer, the more chance that it’s unique; the longer the primer, the higher melting/annealing temperature.
Generally speaking, the length of primer has to be at least 15 bases to ensure uniqueness. Usually, we pick primers of 17-28 bases long. This range varies based on if you can find unique primers with appropriate annealing temperature within this range.
Base Composition
Base composition affects hybridization specificity and melting/annealing temperature.
Random base composition is preferred. We shall avoid long (A+T) and (G+C) rich region if possible.
Usually, average (G+C) content around 50-60% will give us the right melting/annealing temperature for ordinary PCR reactions, and will give appropriate hybridization stability. However, melting/annealing temperature and hybridization stability are affected by other factors, which we’ll discuss later. Therefore, (G+C) content is allowed to change.
Melting Temperature
Melting Temperature, Tm – the temperature at which half the DNA strands are single stranded and half are double-stranded.. Tm is characteristics of the DNA composition; Higher G+C content DNA has a higher Tm due to more H bonds.
Calculation
Shorter than 13: Tm= (wA+xT) * 2 + (yG+zC) * 4
Longer than 13: Tm= 64.9 +41*(yG+zC-16.4)/(wA+xT+yG+zC)
(Formulae are from http://www.basic.northwestern.edu/biotools/oligocalc.html)
Annealing Temperature
Annealing Temperature, Tanneal – the temperature at which primers anneal to the template DNA. It can be calculated from Tm .
Internal Structure
If primers can anneal to themselves, or anneal to each other rather than anneal to the template, the PCR efficiency will be decreased dramatically. They shall be avoided. 
However, sometimes these 2。structures are harmless when the annealing temperature does not allow them to take form. For example, some dimers or hairpins form at 30℃while during PCR cycle, the lowest temperature only drops to 60℃.
