Electronic Design - Industry trend and importance of circuit board substrate

1. Continuous innovation of FR-4 plate

In short, the substrate of circuit board mainly includes three raw materials: copper foil, resin and reinforcing material. However, if we further study the current substrate and examine its changes over the years, we will find that the complexity of the substrate content is unimaginable. Due to the increasingly stringent requirements of circuit board manufacturers for the quality of substrate in the lead-free era, the performance and specifications of resin and substrate will undoubtedly become more complex. The challenge for substrate suppliers is to find the best balance between various customer needs in order to obtain the most economic production benefits, and provide their product data to the overall supply chain as a reference.

Taking a comprehensive view of the development history of FR-4 plate, over the years, some operators have always believed that FR-4 plate has been exhausted, so they turn to other high-performance substitutes. Every time the specification requirements are increased, the plate supplier must work hard to meet the needs of customers. In recent years, the most obvious development trend of the market is the large increase in the demand for high Tg plates. In fact, the understanding of many operators on TG issues seems to indicate that high Tg has high efficiency or better reliability. One of the main purposes of this paper is to explain that the characteristics required for the next generation FR-4 plate can not be fully expressed by TG, so it is to put forward more new specifications for strong heat resistance to meet the challenge of lead-free welding.

2.Industry trends leading substrate specifications

A number of ongoing industrial trends will promote the market and adoption of reformulated plates. These trends include multi-layer plate design trend, environmental protection regulations and electrical demand, which are described below:

2.1. Design trend of multi plate

At present, one of the design trends of PCB is to improve the wiring density. There are three methods to achieve this goal: first, reduce the line width and line distance, so that more and more dense wiring can be accommodated per unit area; The second is to increase the number of circuit board layers; Finally, the pore diameter and the size of the welding pad are reduced.

However, when more lines are distributed per unit area, the operating temperature is bound to rise. Moreover, with the increasing number of circuit board layers, the finished board is bound to thicken synchronously. Otherwise, it can only be pressed 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 heat accumulation will increase, which will increase the thermal expansion effect in Z direction. When selecting a thinner dielectric layer, it means that the substrate and film with more glue content must be used; However, if the glue content is higher, the thermal expansion and stress in the Z direction of the through hole will increase again. In addition, reducing the diameter of the through hole inevitably increases the aspect ratio; Therefore, in order to ensure the reliability of plated through holes, the substrate must have lower thermal expansion and better thermal stability.

In addition to the above factors, when the density of circuit board assembly components increases, the layout of through holes will be arranged more closely. However, this will make the leakage of glass bundle more tense, and even bridge in the base glass fiber between the hole walls, resulting in short circuit. This anodic filamentous leakage phenomenon (CAF) is one of the topics concerned about plates in the lead-free era. Of course, the new generation of substrates must have better CAF resistance to avoid frequent conditions in lead-free welding.

2.2 environmental protection regulations

Environmental regulations add many additional requirements for substrates with political intervention, such as RoHS and WEEE directives of the European Union, which will affect the formulation of plate specifications. In many regulations, ROHS limits the lead content during welding. Tin lead solder has been used in assembly plants for many years. The melting point of its alloy is 183 degree Celsius, while the temperature of fusion welding process is generally about 220 degree Celsius. Tin silver copper alloy of lead-free mainstream solder (such as sac305) has a melting point of about 217 degree Celsius, and usually the peak temperature during fusion welding will be as high as 245 degree Celsius. The rise of welding temperature means that the substrate must have better thermal stability to withstand the thermal shock caused by multiple fusion welding.

RoHS directive also prohibits some halogen containing flame retardants, including polyodorous biphenyl PBB and PBDE. However, the most commonly used flame retardants in PCB substrates, tetraodorylbisphenol TBBA, is not on the RoHS blacklist. Nevertheless, due to the improper ashing reaction of plates containing TBBA when heating up, some machine brands still consider adopting halogen-free materials.

2.3 electrical requirements

The application of high-speed, broadband and radio frequency forces the plate to have better electrical performance, that is, the dielectric constant DK and loss factor DF, which must not only be kept low, but also be stable in the whole board, and should be well controllable. Those who meet these electrical requirements have to be inferior in thermal stability at the same time. Only in this way can their market demand and share be obtained With increasing.

3.Important properties of substrate

In order to consider the heat stability required by the lead-free market, the physical properties that must be paid attention to include: glass transition temperature (TG), thermal expansion coefficient CTEs, and cracking resistance temperature TD required for high-temperature lead-free welding,

The glass transition temperature is an important index most commonly used to evaluate the properties of resin substrates. The so-called Tg of resin refers to that when the polymer is heated to a certain temperature range, the resin will change from the hard "glass state" (a general term for non fixed solid substances) at room temperature to the plastic and soft "rubber state" at high temperature The various properties of various plates before and after TG will be quite different.

All substances will have expansion and contraction changes due to temperature changes. The thermal expansion rate of the substrate before TG is usually low and moderate. Thermal mechanical analysis (TMA) The change of substrate size corresponding to temperature can be recorded. By extrapolation, the intersection of the dotted line extended by the two curves can be used to indicate the temperature, which is the Tg of the substrate. The great difference in the slope of the curve before and after TG shows the different thermal expansion rates of the two, that is, the so-called α 1 and α 2 coefficient of thermal expansion (CTEs) Since the z-cte of the plate will affect the reliability of the finished plate and is more important for downstream assembly, it can not be ignored by all operators. It should be noted that the through-hole copper wall with small thermal expansion will also show less stress, so the reliability must also be better. However, it is generally believed that TG is a fairly fixed temperature point. In fact, it is not According to the curve radian, when the temperature of the plate rises near TG, its physical properties will begin to change greatly.

Figure 1. This is the description of TMA for measuring Tg of the sample. When the z-axis plate thickness gradually increases during sample warming, when the thermal expansion curve changes from room temperature glassy state α- 1cte slope, transition to high temperature rubber state α- For 2cte slope, the temperature range corresponding to the transition state is TG

In addition to TMA test method, there are also differential scanning calorimetry (DSC) and dynamic thermal engine analysis TG can be measured in two ways. Different from TMA, DSC analysis measures the heat flow of the plate corresponding to the change of temperature. Endothermic or exothermic reaction will change the temperature increase of the resin in the TG range. As for TG measured by DSC, it is usually about 5 degree Celsius higher than TMA measurement results. DMA of another dynamic thermomechanical analysis method is to measure the relationship between plate modulus and temperature It will be higher than 15 degree Celsius, and the IPC specification is more consistent with the measurement of TMA.

In addition to measuring the Tg of the finished plate, the above TMA thermal analysis instrument can also place the finished plate in its high-temperature test dish and monitor the heat-resistant cracking time of various finished plates in the Z direction in the set high-temperature environment of 260 degree Celsius, 288 degree Celsius or 300 degree Celsius, referred to as T260, t288 and T300 for short, so as to simulate whether there will be plate burst and crack layer in multiple lead-free welding. At present, ipc-4101b The above three practices have been included in the specification list, which can be regarded as a major reform of FR-4 plate due to lead-free.

3.2 interpretation of coefficient of thermal expansion (CTEs)

Many literatures have indicated that high Tg represents good resin quality, but this is not always the case for lead-free welding. Generally, high Tg will undoubtedly delay the initial temperature before rapid thermal expansion of resin, and its overall thermal expansion varies according to the type of plate. The overall thermal expansion of plate with low Tg is also less. In addition, adding some filler into resin can also reduce the temperature The CTE of the three resin materials shown in Figure 2 above shows that the Tg of material C is higher than that of material a, but the CTE of material C rises rapidly after TG, so the overall thermal expansion is much larger and worse than that of material A. take a and B as an example, if the CTE of the two materials are the same before and after TG, the total thermal expansion of material B with higher Tg will still be lower than that of material A. finally, although the Tg of material B and C are the same, due to B The CTE after TG is low, so the overall thermal expansion of B is relatively small.

It can also be seen that the Tg of the three plates is 175 degree Celsius, but the thermal expansion coefficient of the equal z-axis is different, resulting in the difference of thermal expansion rate. The main difference of the three materials in Figure 3 is the thermal expansion coefficient after TG α- 2cte is different from each other. In a word, the lower the overall thermal expansion coefficient of the plate will help to improve the reliability of the through-hole copper wall.

In fact, this is not always the case! Before we continue to discuss other important properties of the substrate, we must first explain the relationship between TG and CTE. One of the advantages of high Tg plate is that the z-axis thermal expansion coefficient is low, so it has low overall thermal expansion. Therefore, it can delay the adverse phenomenon of rapid thermal expansion after TG and reduce the residual stress in the copper wall.

However, in a few special cases, the CTE of high Tg plate may be larger than that of low Tg plate. Therefore, CTE must be taken into account when selecting plate. Although the Tg of each plate is the same, its CTE may also be different. When thermal cycle test is carried out, the stress felt by through-hole copper wall will also be different. Material C in Figure 3 has the dual advantages of high Tg and low CTE at the same time.