PCB - 1L (Single Sided PCB)
Single sided design can greatly reduce the cost of the board. If you can fit your design on a single sided board then it is preferable to do so. Look inside many of today’s consumer items like TV’s and DVD players, and you will almost certainly find some single sided boards. They are still used because they are so cheap to manufacture. Single sided design however requires some unique techniques which are aren’t required once you go to doubled sided and multi-layer design. It is certainly more challenging than a double sided layout. In fact, a single sided board design will be regarded inversely proportional to the number of jumper links used. No jumper links earns the admiration of many peers!
It is all about a balance between board size and the number of jumper links required. Almost every single sided board will require some jumper links, so it is important to minimise these. Component placement is even more critical on a single sided board, so this is no time to make all your components nice and neatly aligned. Arrange your components so that they give the shortest and most efficient tracking possible. It is like playing a game Chess, if you don’t think many moves ahead then you will get yourself in a corner pretty quickly. Having just one track running from one side of your board to the other can ruin your whole layout, as it makes routing any other perpendicular tracks impossible.
Many people will route their board as though it is a double sided board, but only with straight tracks on the top layer. Then when the board is to be manufactured, the top layer tracks are replaced with jumper links. This can be a rather inefficient way to approach single sided design, and is not recommended. You must be frugal in your placement, and don’t be afraid to rip everything up and try again if you see a better way to route something.
A designer with experience is usually able to specify at the start of a PCB project, if it is possible to route the interconnection structure onto a single electrical layer.
PCB - 2L (Double Sided PCB)
Double sided PCB gives an extra degree of freedom for designing your board. Things that were next to impossible on a single sided board become relatively easy when you add an additional layer. Many (inexperienced) designers tend to become lazy when laying out double sided boards. They think that component placement doesn’t matter all that much, and hundreds of vias can be used to get them out of trouble.
They will often lay out components like ICs in neat rows, and then proceed to route everything using a right angle rules. This means that they will route all the tracks on the bottom layer in one direction, and then all the tracks on the top layer perpendicular to the bottom layer. The theory is that if you chop and change between layers enough times you can route almost anything using a "step" type pattern. This technique can be ugly and inefficient, and is a throw back to the old manual tape days. Many basic auto routers work in this way.
Stick to using good component placement techniques and efficient building block routing.
Double sided design can also give you the chance to make use of good ground plane techniques, required for high frequency designs.