Fabbricazione PCB di precisione, PCB ad alta frequenza, PCB ad alta velocità, PCB standard, PCB multistrato e assemblaggio PCB.
La fabbrica di servizi personalizzati PCB e PCBA più affidabile.
Tecnologia PCB
Analisi e miglioramento del piatto di scoppio nella saldatura di riflusso del bordo PCB
Tecnologia PCB
Analisi e miglioramento del piatto di scoppio nella saldatura di riflusso del bordo PCB

Analisi e miglioramento del piatto di scoppio nella saldatura di riflusso del bordo PCB

2021-12-24
View:163
Author:pcb

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

With the development of electronic products in the direction of multi-function, alta densità, miniaturization, e tridimensionale Sempre più, the need for heat dissipation is becoming more and more important. Allo stesso tempo, the thermal stress and warping caused by the different CTE of many materials make the risk of assembly failure increasing, e la successiva probabilità di guasto precoce dei prodotti elettronici aumenterà anche. Come bigger. Pertanto, PCB soldering reliability has become more and more important. Quanto segue introduce il fenomeno di guasto della piastra nella saldatura a riflusso e i suoi metodi di miglioramento per il vostro riferimento.

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 PCB multistrato 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, e alcuni dei circuiti di secondo livello sono stati strappati a pezzi.

PCB board

1.2 Factors that affect the board burst

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

‘  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, rivelare che l'esistenza di vapore acqueo è la causa primaria dell'esplosione del PCB. 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, più veloce è il tasso iniziale di assorbimento dell'acqua, 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, maggiore è l'assorbimento dell'acqua di equilibrio. In summary, il tasso iniziale di assorbimento dell'acqua della resina epossidica è determinato dal volume libero, while the equilibrium water absorption is determined
by the content of polar groups. On the one hand, la temperatura della scheda PCB aumenta durante la saldatura a riflusso senza piombo, 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, e la frazione volumetrica molare dell'acqua nei vuoti aumenta.

On the other hand, man mano che la temperatura di saldatura aumenta, 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. Quando la forza di legame tra gli strati del materiale è inferiore alla pressione satura del vapore generata dal vapore acqueo, the
material bursts. Therefore, L'assorbimento di umidità prima della saldatura è uno dei motivi principali per la delaminazione del PCB e la rottura della scheda.

‘¡ The influence of moisture during storage and production. PCB multistrato HDI è un componente sensibile all'umidità, 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. Figura 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, con il passare del tempo in posizione statica, 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.

‘¢ 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, con conseguente diminuzione della forza di legame tra gli strati della tavola, and the thermal shock resistance of the board is weakened. Le schede multistrato sono soggette a macchie bianche, bubbling, separazione degli strati in bagni di olio caldo o saldatura e livellamento dell'aria calda.

‘µ Poor adhesion between PP and copper foil is a sufficient condition for plate burst
‘  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, così tanti adesivi hanno pochissima adesione alla lamina di rame. If the surface of the copper foil is not treated, anche se viene utilizzato l'adesivo con ottime prestazioni, 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, anche se l'adesione è aumentata a temperatura ambiente, peeling occurs near 200°C. Ciò è dovuto al fatto che Cu2O non è stabile al calore e si stacca dal foglio di rame dopo il riscaldamento. 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. A metà degli anni '90, 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, che è stato ampiamente utilizzato nel settore.

‘¡ 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-ecching effect of H2O2, la superficie del substrato rame è formata con microstrutture irregolari, 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 che è chemically bonded to the surface of the copper substrate is deposited on the copper substrate, e l'immagine SEM della superficie di rame del substrato è marrone. And after the adhesive
enters the concave-convex part, aumenta anche l'effetto di coinvolgimento meccanico.

‘¢ 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, resistenza agli acidi e agli alcali, corona resistance and high temperature resistance are related to the structure and thickness of the film .
Ma non è che più spessa è la forza del legame, 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, e si è scoperto che lo strato di brunitura era contaminato, 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, e la scheda PCB blister nel successivo assemblaggio SMT. 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.

Ԧ Inappropriate selection of reflow temperature is the predisposing factor for plate burst
‘  The inducing effect of temperature on plate bursting. Through the analysis of sufficient and necessary conditions for the pattern of plate bursting, si può sapere che sono tutte funzioni di temperatura. The amount of volatile matter in the
multilayer board and its expansion pressure increase with the increase of the reflow soldering temperature, mentre l'adesione tra lo strato brunituro e il PP diminuisce con l'aumento della temperatura. Ovviamente, 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.
‘¡ How to optimize the reflow soldering temperature according to product characteristics
(a) US Microelectronics Packaging CG Woychik pointed out: "Using normal SnPb alloy, la temperatura che i componenti e le schede PCB possono sopportare durante la saldatura a riflusso è 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. Soprattutto per molti materiali strutturali laminati, it is easy to cause delamination between layers, especially those new materials
that contain more moisture. The interior contains moisture. In combinazione con l'aumento della temperatura, 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, un'adeguata curva di distribuzione della temperatura di saldatura a riflusso è particolarmente importante. He also
pointed out: According to the current production conditions, Le strutture includono le caratteristiche di temperatura dei componenti e del PCB, etc. The peak temperature of lead-free reflow
soldering should be maintained at 235°C. After comprehensive analysis, nella saldatura a riflusso senza piombo dell'HDI PCB multistrato boards, quando viene utilizzata la saldatura SnAgCu Quando si lega, the peak temperature is recommended to be set at 235°C, not
exceeding 245°C. Practice shows that after taking this measure, l'effetto di soppressione sulla rottura della piastra è molto evidente.

‘· Poor escape of volatile matter is a contributing factor to the failure of the PCB board
From the analysis of the slices, quasi tutte le posizioni della lastra di sabbiatura si sono verificate sulla parte coperta da una grande area di lamina di rame sopra il foro sepolto.
The manufacturability of this design is indeed problematic, mainly in the following aspects:
‘´ 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;
‘µ Intensified the unevenness of the surface temperature distribution during reflow welding;
‘¶ It is not conducive to eliminating the thermal stress in the welding process, ed è facile formare concentrazione di stress, which intensifies the separation between the inner layers of the HDI PCB multistrato. Ovviamente, 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

‘´ The mechanism of plate explosion According to the above analysis and summary of the phenomenon of plate explosion, Possiamo studiare e analizzare il processo fisico di esplosione della piastra secondo il seguente modello fisico.
‘  When the working environment temperature is not too high, Il legame tra le schede multistrato L1-L2 è buono.
‘¡ As the heating process progresses, the volatile matter (including moisture) in the buried hole and the inner layer is continuously discharged.
‘¢ The exhausted volatile gas accumulates between the buried hole and the PP (bonding sheet).
# Mentre la temperatura continua a salire #, more and more gas accumulates near the buried orifice, formare una grande pressione di espansione, che fa sì che la superficie di brunitura di L2 e PP riceva una forza di espansione che li separa.
‘¤ When the final expansion pressureis less than the adsorption force between the browning surface and the PP (f‘¥ When the final expansion pressure is greater than the adsorption force between the browning surface and PP (f>F), La separazione avviene tra la superficie di brunitura e PP lungo L2, come mostrato nella figura 1.14 This kind of obvious lumpy bubbling
and stratification phenomenon. While the PCB is heated, parte del volume libero di acqua può essere persa attraverso il substrato PCB microporoso, riducendo così la frazione volumetrica molare dell'acqua che può accumularsi nei vuoti o nei microcircuiti,
che favorisce il fallimento del PCB. improve. Tuttavia, se la superficie del PCB è coperta da una grande area di foglio di rame, when the PCB is heated, the large copper foil surface above the buried hole blocks the water vapor that escapes
after heating, che aumenterà la pressione del vapore acqueo nei microcrack, causing the occurrence of The chances of breaking the board are greatly increased.

1.4 Countermeasures to prevent board explosion
‘´ 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. Al fine di mantenere inalterate le prestazioni originali del PP, the more suitable storage
conditions are: temperature (10-20) , humidity <50%RH (stored in a vacuum). Secondo le relazioni, the adhesive sheets stored at 5°C for a month or lonhttps://www.ipcb.com/pcb-board.htmlger cannot successfully produce high-quality multilayer boards, so refrigeration is not
advisable. Strictly control the warehouse storage conditions of PCB board products, soprattutto in caso di pioggia, 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.

‘µ 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. Garantire l'uniformità e il tasso di occupazione della resina esistente nello spazio della fibra impregnata, per garantire che il materiale del substrato formato abbia un basso assorbimento d'acqua, better dielectric properties, buona adesione all'intercalare e stabilità dimensionale.

¶ Migliorare la permeabilità all'aria della grande superficie di lamina di rame. Secondo l'analisi di cui sopra delle caratteristiche di posizione dell'esplosione della piastra e del meccanismo dell'esplosione della piastra. Obviously, when the PCB surface has a large area of
copper foil layer design, farà sì che il vapore acqueo interno non sia in grado di rilasciare, 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.
‘· Optimize the peak temperature of reflow welding. Under the condition of ensuring good wetting, ridurre il più possibile la temperatura di picco di riflusso.