PCB News - The industry trend and importance of circuit board substrates

The industry trend and importance of circuit board substrates

1. Continuous innovation of FR-4 board
In short, circuit board substrates mainly include three major raw materials: copper foil, resin, and reinforcing materials. However, if you further study the current substrate and examine its changes over the years, you will find that the complexity of the substrate content is really unimaginable. As circuit board manufacturers have increasingly stringent requirements for the quality of substrates in the lead-free era, the performance and specifications of resins and substrates will undoubtedly become more complex. The challenge faced by substrate suppliers is to find the best balance between the various needs of customers in order to obtain the most economical production benefits, and provide their product data to the overall supply chain as a reference.

Looking at the development history of FR-4 sheet, for many years, some industry players have always believed that FR-4 sheet will be exhausted, so they have turned to other high-performance alternatives. Every time when the specification requirements increase, the sheet metal supplier must work hard to meet customer needs. In recent years, the most obvious development trend in the market has been the huge increase in demand for high Tg sheets. In fact, the understanding of many industry players on Tg issues seems to indicate that high Tg has high performance or better reliability. One of the main purposes of this article is to explain that the characteristics required by the next generation of FR-4 sheets are no longer fully expressed by Tg. Therefore, more new specifications for strong heat resistance are proposed to respond to lead-free soldering. Challenge.

2. Industry trends leading the specifications of substrates
A number of ongoing industrial trends will promote the market application and adoption of reformulated panels. These trends include the design trend of multi-layer panels, environmental protection regulations, and electrical requirements, which are described below:

2.1. Design trend of multi-wide board
One of the current PCB design trends is to increase the wiring density. There are three ways to achieve this goal: firstly, reduce the line width and line spacing, so that more and denser wiring can be accommodated per unit area; secondly, increase the circuit board The number of layers; the last is to reduce the aperture and the size of the solder pad.

However, when there are more lines per unit area, its operating temperature is bound to rise. Furthermore, as the number of layers of circuit boards is continuously increased, the finished boards will inevitably become thicker at the same time. Otherwise, it can only be laminated with a thinner dielectric layer to maintain the original thickness. The thicker the PCB, the more the thermal stress of the through-hole wall caused by the heat accumulation will increase, which will increase the thermal expansion effect in the Z direction. When a thinner dielectric layer is selected, it means that a substrate and film with a higher glue content must be used; but a higher glue content will cause the thermal expansion and stress in the Z direction of the through hole to increase. In addition, reducing the aperture of the through hole will inevitably increase the aspect ratio; therefore, in order to ensure the reliability of the plated through hole, the substrate used must have lower thermal expansion and better thermal stability so as not to fall short.
In addition to the above factors, when the density of the assembly components of the circuit board increases, the layout of the via holes will also be arranged more closely. However, this action will make the leakage of the glass bundle more tense, and even bridge the substrate glass fiber between the hole walls, which will lead to a short circuit. This kind of anodic filamentary leakage phenomenon (CAF) is one of the themes of the current lead-free era for sheet metal. Of course, the new generation of substrates must have better anti-CAF ability to prevent frequent occurrences in lead-free soldering. .

2.2. Environmental protection laws and regulations
Environmental protection regulations have added many additional requirements for substrates under political intervention. For example, EU directives such as RoHS and WEEE will affect the formulation of sheet material specifications. Among many regulations, RoHS limits the lead content during soldering. Tin-lead solder has been used in assembly plants for many years. The melting point of its alloy is 183°C, and the temperature of the fusion soldering process is generally about 220°C. Lead-free mainstream solder tin-silver-copper alloys (such as SAC305 has a melting point of about 217°C, and usually peak temperature during fusion soldering will be as high as 245°C. The increase in soldering temperature means that the base material must have better thermal stability before it can be tolerated. Thermal shock caused by multiple fusion welding.

The RoHS directive also bans certain halogen-containing flame retardants, including PBB and PBDE. However, TBBA, the most commonly used flame retardant in PCB substrates, is actually not on the RoHS blacklist. Nevertheless, due to the improper ashing reaction of TBBA-containing plates when the temperature is raised, some brand manufacturers of the whole machine are still considering changing to halogen-free materials.

2.3. Electrical requirements
The application of high-speed, broadband, and wireless radio frequency forces the board to have better electrical performance, that is, the dielectric constant Dk and the dissipation factor Df must not only be suppressed, but also have a stable performance across the board, and it should also be appropriate. Prepared for controllability. Those who meet these electrical requirements also have to be inferior in thermal stability. Only in this way, their market demand and market share can increase day by day.

3. The important characteristics of the base material, the circuit board manufacturerIn order to take into account the heat-resistant stability required by the lead-free market, the physical properties that must be paid attention to are: glass transition temperature (Tg), thermal expansion coefficient CTEs, and cracking resistance temperature Td that is newly required for high-temperature lead-free soldering. Described below:

3.1. Measure the glass transition temperature (Tg) by the TMA method
The glass transition temperature is an important indicator most commonly used to judge the characteristics of resin substrates. The so-called Tg of the resin means that when the polymer is heated to a certain temperature range, the resin will change from the original "glassy state" (general term for non-fixed composition solid substance), which is relatively hard at room temperature, to plasticity at high temperature. And the softer "rubbery state." The various properties of various sheets before and after Tg will be completely different.

All substances will undergo expansion and contraction changes due to temperature changes, and the thermal expansion rate of the substrate before Tg is usually lower and gentler. The thermal mechanical analysis method (TMA) can record the change in the size of the substrate corresponding to the temperature. Using the extrapolation method, the intersection of the dashed lines extended by the two curves can be used to indicate the temperature, which is the Tg of the substrate. This huge difference in the slopes of the curves before and after the Tg shows that the two have completely different thermal expansion rates, the so-called coefficients of thermal expansion (CTEs) of α1 and α2. Since the Z-CTE of the board will affect the reliability of the finished board, and it is more important for downstream assembly, it should not be ignored by all manufacturers. It should be noted that the lesser thermal expansion exhibits less stress on the copper wall of the through hole, so the reliability must be better. However, most people always think that Tg is a fairly fixed temperature point. In fact, it is not. When the temperature rises near the Tg, the physical properties of the sheet will begin to change significantly.