PCB smt's overall requirements for printed circuit boards

Surface Mount Printed Circuit Board (SMB) is very different from traditional printed circuit boards. The PCB SMT reflow process requires SMB to pass through a 230 ° C (lead-free up to 260 ° C) high temperature furnace. Therefore, the requirements of the PCB SMT substrate material are much higher than those of the conventional printed circuit board, and the requirements are mainly high temperature resistance and flatness. In addition, because SMB has the characteristics of high density and small aperture, the processing difficulty is much larger than that of traditional printed circuit boards.

The requirements of PCB SMT for printed circuit boards are as follows.

1. The appearance dimension is stable, and the warpage is less than 0.0075mm/mm (the ultra-high density warpage is required to be controlled within 0.5%).

2. The pad plating is flat, meeting the SMD coplanarity requirements.

3. The coefficient of thermal expansion is small and the thermal conductivity is high.

4. Copper foil has high adhesion strength and good solderability.

5. High anti-bending strength.

6. Electrical performance requirements: dielectric constant, withstand voltage, insulation performance should meet product requirements.

7.High-density PCBs such as GBA and CSP use multi-layer boards with buried or blind holes.

8. Washable.

It’s selection of surface assembly PCB material

1. Select the PCB SMT according to the function, performance index and grade of the product.

2. For general electronic products, FR-4 epoxy glass fiber substrate is used; lead-free high-Tg (150-1700C) FR-4, CEM-3, FR-5, etc. can be selected.

3. For the use of high requirements of ambient temperature or flexible circuit boards, polyimide fiberglass substrates are used.

4. For high-reliability boards with high heat dissipation requirements, metal substrates are used.

5. For high frequency circuits, a Teflon glass fiber substrate is required.

FR-4 characteristics for lead-free soldering

1.Tg>145 ° C (typically 150-170 ° C).

2.Decomposition temperature is high (358 ° C).

3. Excellent dimensional stability in the hot state.

4.Z axis expansion coefficient is small.

5.Ion migration resistance is good.