PRINTED CIRCUIT BOARD (PCB) SURFACE FINISH COMPARISON BASIC INFORMATION AND TUTORIALS

For making sound interconnections, the characteristics and properties of the circuit board surface finish are as important as the component leads and termination. Hot air solder leveled SnPb (HASL) has been used successfully as the surface finish for surface mount and mixed PCBs.

As the need of a flat surface with uniform thickness becomes increasingly important to forming consistent and reliable fine-pitch solder joints, the HASL process often falls short.

Alternatives to HASL include immersion Sn, electroplated SnPb (reflowed or nonreflowed), electroplated Au/Ni, electroless Au/electroless Ni, immersion Au/ electroless Ni, immersion Pd, immersion Pb/electroless Ni, electroplated Sn- Ni alloy, and organic coating.

When selecting an alternative surface finish for a PCB assembly, the key parameters to consider are solderability, ambient stability, high-temperature stability, suitability of the use as contact/switch surface, solder-joint integrity, wire bondability of assemblies that involve wire bonding, and cost.

Whatever its deficiencies may be, HASL provides the most solderable surface. However, comparing a metallic system with HASL, the latter subjects PCBs to higher temperatures (above 200°C), producing inevitable thermal stress in PCBs.

Furthermore, HASL is not suitable for wire bonding. To choose a valid replacement for HASL, many variables need to be assessed. Understanding the fundamentals behind each variable, in conjunction with setting proper priority of importance among these variables for a specific application, is the way to reach the best balanced solution.

When selecting an alternative surface finish for PCB assembly, the key parameters to consider are solderability, ambient stability, high temperature stability, suitable for use as a contact/switch surface, solder-joint integrity, and wire bondability for those assemblies that involve wire bonding, and cost.

Table 5.14 summarizes the relative performance of PCB surface finish systems.

BREADBOARD AND PRINTED CIRCUIT BOARD BASICS AND TUTORIALS

WHAT ARE BREADBOARD AND PRINTED CIRCUIT BOARD?

A breadboard is a rectangular plastic box filled with holes, which have contacts in which you can insert electronic components and wires. A breadboard is what you use to string together a temporary version of your circuit.

You don’t have to solder wires or anything else; instead, you poke your components and wires into the little contact holes arranged in rows and connected by lines of metal; then you can connect your components together with wires to form your circuit.

The nice thing about breadboards is that you can change your mind and replace or rearrange components as you like. You typically create an electronics project on a breadboard to make sure that everything works.

If it’s a project you wish to save, you can create a more permanent version.  If you want to create a permanent version of your circuit, you need to create a soldered or printed circuit board; see the sidebar, “Printed circuit boards,” to find out how to go about that.


Printed circuit boards 
If you create a circuit on a breadboard and decide that it’s worthy of immortality, you can make it permanent by soldering components in place on a printed circuit board. To do this, you have to get your hands on a universal printed circuit board.

This is much like a breadboard except that you can solder all the connections you’ve made to keep them around. A universal printed circuit board has rows of individual holes throughout the board with copper pads around each hole and metal lines connecting the holes in each row, like in a breadboard.

You mount parts on the face of the board and then pass leads through holes to the components. You can solder the leads to the copper pads on the bottom of the board.

Universal printed circuit boards are available in a variety of patterns of contact holes and
metal lines. The figure here shows one we like because there are rows on either side that accommodate discrete components handily.

You can get custom printed circuit boards made for your circuit; this is typically done by submitting a drawing of your circuit to a printed circuit board company. These boards eliminate the need to solder jumper wires between components.