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Analysis and improvement of burst plate in PCB board reflow welding
PCB Tech
Analysis and improvement of burst plate in PCB board reflow welding

Analysis and improvement of burst plate in PCB board reflow welding


Preface, in PCB board reflow welding analysis and improvement of burst plate

With the development of electronic products in the direction of multi-function, high density, miniaturization, and three-dimensional More and more, the need for heat dissipation is becoming more and more important. At the same time, the thermal stress and warping caused by the different CTE of many materials make the risk of assembly failure increasing, and the subsequent probability of early failure of electronic products will also increase. Come bigger. Therefore, PCB soldering reliability has become more and more important. The following introduces the phenomenon of plate failure in reflow welding and its improvement methods for your reference.

1. Plate burst phenomenon in reflow welding

1.1 Definition of burst board
Definition: In the process of reflow soldering (especially lead-free applications), the separation phenomenon that occurs between the PP layer and the secondary layer (L2) copper foil brown surface of the HDI multilayer PCB during the second
compression, we Define it as a burst board. From the analysis of the slices, the locations of the board bursts occurred in the densely buried areas of the 1-2 layers; no debris or other abnormalities were found; the slices showed that the board bursts very violently, and some of the second-layer circuits were pulled apart.

PCB board

1.2 Factors that affect the board burst

⑴ The formation source of volatile matter is a necessary condition for the explosion

① Moisture absorption problem
The following shows that the existence of water in the PCB board, the way of water vapor diffusion and the change of water vapor pressure with temperature, to reveal that the existence of water vapor is the primary cause of PCB explosion. The
moisture in the PCB mainly exists in the resin molecules, as well as the macro-physical defects (such as voids, micro-cracks) inside the PCB board. The water absorption rate and equilibrium water absorption of epoxy resin are mainly determined by
the free volume and the concentration of polar groups. The greater the free volume, the faster the initial water absorption rate, and the polar groups have an affinity for water, which is the main reason why epoxy resins have higher water
absorption. The greater the content of polar groups, the greater the equilibrium water absorption. In summary, the initial water absorption rate of epoxy resin is determined by the free volume, while the equilibrium water absorption is determined
by the content of polar groups. On the one hand, the temperature of the PCB board increases during lead-free reflow soldering, which causes the water in the free volume and the polar group to form hydrogen bonds, which can obtain enough
energy to diffuse in the resin. The water diffuses outward and gathers in the voids or microcracks, and the molar volume fraction of water in the voids increases.

On the other hand, as the welding temperature increases, the saturated vapor pressure of water also increases. The saturated vapor pressure of water vapor at 224°C is 2500kPa; the saturated vapor pressure of water vapor at 250°C is 4000kPa; and
when the welding temperature rises to 260°C, the saturated vapor pressure of water vapor even reaches 5000kPa. When the bonding strength between the material layers is lower than the saturated vapor pressure generated by water vapor, the
material bursts. Therefore, moisture absorption before soldering is one of the main reasons for PCB delamination and board burst.

② The influence of moisture during storage and production. HDI multilayer PCB is a moisture-sensitive component, and the presence of water in the PCB has an extremely important effect on its performance. For example:
(a) The moisture in the storage environment will cause significant changes in the characteristics of PP (prepreg);
(b) Without protection, PP is very easy to absorb moisture. Figure 1.3 shows the moisture absorption of PP when stored under the conditions of 30%, 50%, and 90% relative humidity; the relationship between the storage time and moisture
absorption rate of PP The relationship is obvious, with the passage of time under static placement, the moisture content of the PCB board will gradually increase. The water absorption rate of vacuum packaging is higher than the water absorption
rate of no vacuum packaging, and the difference in water absorption rate with the increase of temporary storage time.
(c) Moisture mainly penetrates the interface between various substances in the resin system, and there is an impact of water on the interface.

③ Harm of moisture absorption
(a) Increase the volatile content of PP.
(b) The presence of moisture in the PP resin weakens the cross-linking between the resin molecules, resulting in a decrease in the bonding force between the layers of the board, and the thermal shock resistance of the board is weakened. Multi-layer boards are prone to white spots, bubbling, and layer separation in hot oil or solder baths and hot air leveling.

⑵ Poor adhesion between PP and copper foil is a sufficient condition for plate burst
① Phenomenon description It can be seen from the slice analysis that the position of the plate burst is between the secondary pressing pp and the contact surface (browning surface) of the copper foil.
Copper is a non-polar substance in the metallic state, so many adhesives have very little adhesion to copper foil. If the surface of the copper foil is not treated, even if the adhesive with excellent performance is used, it will not have sufficient
adhesion and heat resistance. The early browning treatment method on the surface of copper foil was to form reddish-brown cuprous oxide (Cu2O) on the surface of the copper foil through chemical treatment. When it is bonded to a resin-
laminated substrate board, although the adhesion is increased at room temperature, peeling occurs near 200°C. This is due to the fact that Cu2O is not stable to heat and peels off from the copper foil after heating. In the 1960s, researchers at
Toshiba Corporation in Japan discovered that after treatment with a special chemical solution, the black velvet-like film (CuO) formed on the surface of the copper foil has finer crystals and can firmly adhere to the surface of the copper foil. The
stability is also very good, this is the blackening process commonly used later. In the mid-1990s, Europe and the United States used a new type of browning process in which the inner conductive pattern of a new multilayer board was chemically
oxidized to replace the traditional blackening process, which has been widely used in the industry.

② Mechanism of browning enhancing adhesion
The new browning process, its chemical reaction mechanism is: 2Cu + H2SO4 + H2O2 + nR1 + nR2 → CuSO4 + 2H2O + Cu(R1 + R2)
In the browning tank, due to the micro-etching effect of H2O2, the surface of the substrate copper is formed with uneven microstructures, so the bonding area equivalent to 6-7 times the untreated smooth copper surface can be obtained. At the
same time, a thin layer of organic metal film that is chemically bonded to the surface of the copper substrate is deposited on the copper substrate, and the SEM image of the copper surface of the substrate is browned. And after the adhesive
enters the concave-convex part, it also increases the mechanical engagement effect.

③ Factors affecting the browning effect
The quality and effect of browning depend on the refinement of the process parameters control, such as:
(a) Choose a potion with advanced formula:
The browning layer with Atotech potion has a large roughness, and the binding force of the browning layer can withstand 12 times of lead-free reflow temperature without breaking the board.
(b) Strengthen the monitoring of bath liquid composition during the production process.
(c) Browning (or black copper oxide) film thickness: Browning (or black copper oxide) film and PP bonding strength, acid and alkali resistance, corona resistance and high temperature resistance are related to the structure and thickness of the film .
But it is not that the thicker the bond strength, the higher.
(d) Contaminated browning layer and process error: In the quality of a cracked plate, peel off the part where the cracked plate occurred, and it was found that the browning layer was contaminated, and the resin was completely separated from the
contaminated browning layer.
The browning layer of the contaminated part and the pp sheet failed to effectively bond after lamination, and the PCB board blistered in the subsequent SMT assembly. After investigation, the high Tg material was misused to press and solidify the
common material, which is also one of the reasons for the poor bonding between the outer copper foil and the pp sheet.

⑶ Inappropriate selection of reflow temperature is the predisposing factor for plate burst
① The inducing effect of temperature on plate bursting. Through the analysis of sufficient and necessary conditions for the pattern of plate bursting, it can be known that they are all functions of temperature. The amount of volatile matter in the
multilayer board and its expansion pressure increase with the increase of the reflow soldering temperature, while the adhesion between the browning layer and the PP decreases with the increase of temperature. Obviously, the sufficient and
necessary conditions for the latent explosion board must be induced by the factor of temperature. Optimizing the reflow welding temperature curve based on the comprehensive analysis of specific product characteristics is effective in suppressing

the occurrence of plate bursting.
② How to optimize the reflow soldering temperature according to product characteristics
(a) US Microelectronics Packaging CG Woychik pointed out: "Using normal SnPb alloy, the temperature that components and PCB boards can withstand during reflow soldering is 240℃. When using SnAgCu (lead-free) alloy, JEDEC stipulates The
temperature is 260°C. The increase in temperature may endanger the integrity of the electronic packaging assembly. Especially for many laminated structural materials, it is easy to cause delamination between layers, especially those new materials
that contain more moisture. The interior contains moisture. Combined with the increase in temperature, most of the commonly used laminates (HDI multi-layer PCB boards) will have a large range of delamination."
(b) The American electronic assembly welding JSHwang also has this description in the book "Welding materials and processes in electronic assembly manufacturing": "Considering that the melting point temperature of existing lead-free materials is higher than that of SnPb eutectic materials Melting point temperature (183℃), in order to reduce the reflow soldering temperature to a certain degree, a suitable reflow soldering temperature distribution curve is particularly important. He also
pointed out: According to the current production conditions, such as existing SMT manufacturers and foundations The facilities include the temperature characteristics of components and PCB, etc. The peak temperature of lead-free reflow
soldering should be maintained at 235°C. After comprehensive analysis, in the lead-free reflow soldering of HDI multilayer PCB boards, when SnAgCu solder is used When alloying, the peak temperature is recommended to be set at 235°C, not
exceeding 245°C. Practice shows that after taking this measure, the suppression effect on the plate burst is very obvious.

⑷ Poor escape of volatile matter is a contributing factor to the failure of the PCB board
From the analysis of the slices, almost all the locations of the blasting plate occurred on the part covered with a large area of copper foil above the buried hole.
The manufacturability of this design is indeed problematic, mainly in the following aspects:
⑴ After the welding is heated, it is unfavorable for the emission of the volatile matter (such as moisture, etc.) accumulated in the buried hole and the interlayer;
⑵ Intensified the unevenness of the surface temperature distribution during reflow welding;
⑶ It is not conducive to eliminating the thermal stress in the welding process, and it is easy to form stress concentration, which intensifies the separation between the inner layers of the HDI multilayer PCB. Obviously, the unreasonable graphic design of HDI multilayer board products has contributed to the occurrence of board bursts in the lead-free manufacturing process.

1.3 The mechanism of board burst

⑴ The mechanism of plate explosion According to the above analysis and summary of the phenomenon of plate explosion, we can study and analyze the physical process of plate explosion according to the following physical model.
① When the working environment temperature is not too high, the bonding between the multilayer boards L1-L2 is good.
② As the heating process progresses, the volatile matter (including moisture) in the buried hole and the inner layer is continuously discharged.
③ The exhausted volatile gas accumulates between the buried hole and the PP (bonding sheet).
④ As the temperature continues to rise, more and more gas accumulates near the buried orifice, forming a large expansion pressure, which makes the browning surface of L2 and PP receive an expansion force that separates them.
⑤ When the final expansion pressure  is less than the adsorption force between the browning surface and the PP (f⑥ When the final expansion pressure is greater than the adsorption force between the browning surface and PP (f>F), separation occurs between the browning surface and PP along L2, as shown in Figure 1.14 This kind of obvious lumpy bubbling
and stratification phenomenon. While the PCB is heated, part of the free volume of water can be lost through the microporous PCB substrate, thereby reducing the molar volume fraction of water that may accumulate in the voids or microcracks,
which is conducive to the failure of the PCB. improve. However, if the PCB surface is covered by a large area of copper foil pattern, when the PCB is heated, the large copper foil surface above the buried hole blocks the water vapor that escapes
after heating, which will increase the pressure of water vapor in the microcracks, causing the occurrence of The chances of breaking the board are greatly increased.

1.4 Countermeasures to prevent board explosion
⑴ Necessary conditions for eradicating plate explosion
The problem in PP storage is to prevent it from absorbing moisture. The moisture in the air is easy to condense on the PP and become adsorbed water. In order to keep the original performance of PP unchanged, the more suitable storage
conditions are: temperature (10-20) ℃, humidity <50%RH (stored in a vacuum). According to reports, the adhesive sheets stored at 5°C for a month or lon cannot successfully produce high-quality multilayer boards, so refrigeration is not
advisable. Strictly control the warehouse storage conditions of PCB board products, especially in rainy weather, increase the power of the dehumidifier to control the humidity in the warehouse; improve the packaging of PCB products used in the
lead-free process, and use vacuum film + aluminum film packaging to ensure Storage time and dryness; look for new materials with good heat resistance and low moisture absorption.

⑵ Sufficient conditions for suppressing the occurrence of board explosion: optimizing the quality of the "browning" process and increasing the adhesion between the internal layers of the PCB; selecting high-quality browning potions;
strengthening the monitoring of the quality of raw materials, such as the resin content of PP materials (RC%), Resin gel time (GT), resin fluidity (RF%), volatile content (VC%) and other key indicators. To ensure the uniformity and occupancy rate of the resin existing in the impregnated fiber space, to ensure that the formed substrate material has low water absorption, better dielectric properties, good interlayer adhesion and dimensional stability.

⑶ Improve the air permeability of the large copper foil surface. According to the above analysis of the location characteristics of the plate explosion and the mechanism of the plate explosion. Obviously, when the PCB surface has a large area of
copper foil layer design, it will cause the internal water vapor to be unable to release, so it is necessary to open a window to the area covered by the large copper surface on the surface to improve the phenomenon of PCB board explosion.
⑷ Optimize the peak temperature of reflow welding. Under the condition of ensuring good wetting, reduce the peak temperature of reflow as much as possible.