Basic Electronic Component Manufacturing

Why Learn Structure If We Don't Make the Baseboard

Most electronics companies in the Gyeonggi, Incheon, and China regions do not produce PCB baseboards (bareboards) themselves. Instead, they receive them and perform the task of mounting components (PCBA). Representative examples include automotive electronics (ECU, sensors, motor drivers) in Changwon and Gimhae, industrial control panels in Busan and Yangsan, and control cards for shipbuilding equipment.

However, there are reasons why you need to understand the structure.

• If the board warps or delaminates, you need to determine whether the issue is with the baseboard or your process to resolve the claim.
• When setting the reflow temperature, the thickness and number of layers determine the thermal capacity. The same profile may work for a 4-layer board but cause cold solder on an 8-layer board.
• The answer to "Why isn't solder sticking to this pad?" is mostly found in the solder mask and surface finish.

What Is a PCB Made Of

FR-4 Is the Standard

FR-4 = Flame Retardant type 4, a flame-retardant material made of glass fiber fabric soaked in epoxy resin. The general standard thickness is 1.6mm, and thinner boards use 0.8–1.2mm.

An important number is Tg (glass transition temperature). If this temperature is exceeded, the board becomes soft.

> With the switch to lead-free solder, reflow peak temperatures have risen to around 245°C. Using low Tg materials with lead-free profiles causes delamination and warping. Check the Tg value first in the material specification sheet.

Copper Foil Thickness Is Expressed in Ounces

1 oz   = 1 ounce of copper per square foot thickness = about 35um
0.5 oz = about 18um   common for inner signal layers
2 oz   = about 70um   used for high current patterns, power sections, motor drivers

Thicker copper foil allows more current to flow, but it makes it harder to reduce the spacing between patterns and reduces etching precision. That's why you don't make it thick everywhere.

Single-Sided, Double-Sided, and Multilayer

The number of layers increases not only due to wiring space. Adding a solid ground plane (GND Plane) reduces noise and stabilizes impedance. That's why high-speed and high-frequency circuits have more layers.

Reading the Stackup

Example of a 4-layer 1.6mm board stackup

  L1  Top       Copper Foil 35um (1oz)      Components and signals
  ------ Prepreg 0.2mm ------
  L2  GND       Copper Foil 18um            Solid ground
  ------ Core 1.1mm ------
  L3  POWER     Copper Foil 18um            Power plane
  ------ Prepreg 0.2mm ------
  L4  Bottom    Copper Foil 35um            Signals and some components

  Insulation layer total = 0.2 + 1.1 + 0.2 = 1.5mm
  Copper + plating total = 35 + 18 + 18 + 35 um = 106um = about 0.1mm
  Total thickness = about 1.6mm

Layer numbers are always counted from the top (component side). If the drawing says L1, it's the top. Ordering layers reversed to the baseboard manufacturer results in full rejection.

Via — Holes Connecting Layers

There are also limits on the aspect ratio (ratio of board thickness to hole diameter).

Drill diameter 0.3mm, board thickness 1.6mm
  Aspect ratio = 1.6 / 0.3 = 5.3 : 1

General mass production limit is approximately 8:1 to 10:1
  Exceeding this causes thin plating in the hole, leading to cracks and open circuits

> Unsealed via-in-pad designs are the most common mistakes made by new designers. When placed on the SMT line, solder paste can fall into the hole, causing cold solder on that component.

Pads and Solder Mask

There are two types based on how the solder mask covers the pad area.

The thin strip of mask left between adjacent pads is called a solder mask dam. If this dam is too thin in fine-pitch applications, it can collapse during printing and reflow, causing a bridge.

Surface Finish — Determines Whether Solder Can Attach

> OSP boards stored for too long in the warehouse will not accept solder. Manage the shelf life (usually several months after opening, check the specification sheet) with labels. "It's just inventory, so just use it" can result in cold solder across the entire line.

Handling — This Is Where Defects Are Created

• Do not touch the pads with bare hands. Salt and oil from fingerprints block soldering, and over time, corrosion occurs. Use gloves (ESD gloves) or only grasp the board's edges.
• Do not stack boards on top of each other. Components on the top board can scratch the pads on the lower board. Use magazine racks or slip sheets.
• Boards absorb moisture. Boards left in vacuum packaging for a long time can experience delamination and bulging during reflow. The issue is more severe for multilayer and thick boards.
• They are ESD sensitive. Bareboards themselves are fine, but boards with components mounted (PCBA) are ESD sensitive products.

First Week Checklist

• Number of layers, thickness, Tg rating of the most frequently produced board on our line
• Surface finish type and shelf life management method for that board
• Storage location and packaging condition of the bareboard (whether vacuum packaging is opened)
• Tried the correct way to hold the board (grasping the edges)
• Confirmed which side L1 is on in the drawing