Development at a rocketing speed of electronics industry lays increasingly higher demands to PCB (printed circuit board) fabrication industry including constantly rising board layers, increasingly high density of tracing and constantly thinner internal layers, all of which leads to increasing significance of layer stackup and lamination technology.
To stop quality issues from taking place during lamination process such as misplacement, fusion is usually called for before layer stackup during the process of multi-layer PCB manufacturing process. Compared with traditional fusion technology, modern fusion technology features advantages of high efficiency, easy operation and low cost, which leads it to be accessible for multi-layer PCB fabrication. Starting from fundamental technologies of fusion PCB fabrication, this article will discuss parameter factors affecting fusion effect and application level of fusion technology and provide a reliable reference with optimal fusion parameters obtained.
Principles of Fusion Technology
As a traditional technology, rivet technology has been widely applied in PCB board fabrication. Nevertheless, rivet technology features some disadvantages as well such as high cost of PCB board due to high cost of rivets, misplacement due to circuit board deformation, stencil's accessibility to damage, rivet-shape indentation on circuit board etc. As a result, fusion technology has been constantly used to replace rivet technology.
Depending on the melting property of epoxy resin prepreg, fusion technology works through getting prepreg melted under a certain temperature so that B-phase epoxy resin is converted into C-phase epoxy resin with internal layers connected through adhesives. Fusion is one of the most important processes during lamination and its performance directly determines behavior of lamination. Key elements concerning fusion technology include:
• Accuracy of Positioning System
The type of positioning system is directly associated with alignment accuracy between internal layers, which further influences percent of pass rate. Excellent positioning systems should be stable, reliable and well repeatable.
• Fusion Point Design
Fusion point is an essential issue as far as fusion technology is concerned with numerous shapes such as square, circle and oval. Fusion point should be agreeable in terms of area since fusion points with too small area tend to lead fusion welding that is not so solid while fusion points with too large area tend to lead image penetration that will possibly cause white spot, loose connection between internal layers or delamination.
• Flatness of Equipment
Equipment flatness influences PCB board angularity during fusion procedure, force distribution during fusion procedure and moment balance. Unevenness will lead circuit board to become deformed, which will further lead to misplacement between layers.
• Temperature and Time Control
In the process of fusion technology implementation, temperature and time should be carefully mastered and controlled to avoid burning, white spot, desoldering and aging. Furthermore, layer stackup of PCB board also plays a significant role in determining fusion effect.
Factors Affecting Fusion Performance in Fusion PCB Fabrication
• Fusion Welding Joint
Different fusion bond and fusion effect are summarized in the following table catering to different types of fusion welding joints.
|
Fusion Welding Joint Shape
|
Between L1/2 and PP
|
Between L3/4 and PP
|
Between L5/6 and PP
|
Average Bond
|
|
Circle |
6.19 |
4.51 |
5.99 |
5.62 |
|
5.81 |
4.82 |
6.07 |
|
6.06 |
5.38 |
5.77 |
|
Rectangle |
9.77 |
7.89 |
9.46 |
8.71 |
|
9.90 |
6.78 |
9.58 |
|
8.75 |
6.94 |
9.32 |
Based on table demonstrated above, because the area of rectangular fusion welding joint is three times larger than that of circular fusion welding joint, bond generated by rectangular fusion welding joint is obviously larger than that generated by circular fusion welding joint. Nevertheless, resin flow generated by rectangular fusion welding joint is much more than that generated by circular fusion welding joint. When resin flow is too large, partial board side will be possibly higher than board, possibly causing virtual pressure on board side. When it comes to small-size PCB products, because designable fusion points are so limited and circular fusion welding joints features small area, fusion bond will be insufficient. As a result, rectangular fusion welding joint should be picked and fusion positions should be carefully designed. With board moved inward suitably, the defect of over resin flow can be defeated.
• Fusion Temperature
When fusion temperature reaches 300°C, fusion expansion area is relatively large with fusion effect badly aroused. When fusion temperature reaches 270°C, fusion expansion area is uneven with crack risk with fusion effect badly aroused. When fusion temperature reaches 285°C, fusion expansion is even with no crack risk, leading to optimal fusion effect. Therefore, it can be concluded that at the same fusion time and layer stackup, 285°C is the best fusion temperature for multi-layer PCB fabrication.
• Fusion Time
At equivalent fusion temperature and layer stackup, different fusion time affects fusion expansion area and fusion effect. When fusion time is 12 seconds, fusion expansion area is uneven with crack risk and badly-performed fusion effect. When fusion time is 18 seconds, fusion expansion area is large with bad fusion effect. When fusion time is 15 seconds, fusion expansion is even with no crack risk and optimal fusion effect. Thus, at equivalent fusion temperature and equivalent layer stackup, 15 seconds is an optimal fusion time for multi-layer PCB fabrication. Either too long or too short fusion time will bring forward bad fusion effect.
• Layer Stackup
At equivalent fusion temperature and fusion time, different layer stackups determine fusion area and fusion effect. At equivalent fusion time and fusion temperature, fusion expansion area is even with no cracks as prepreg 2116 is applied, leading to optimal fusion effect. At equivalent fusion time and fusion temperature, fusion expansion area is even with cracks as prepreg 7628 is applied. It indicates that at equivalent fusion time and fusion temperature, the thinner prepreg is, the better fusion effect will be generated. Therefore, it can be concluded that layer stackup with 2116 prepreg or below is suitable for fusion technology implementation during multi-layer PCB fabrication.
In accordance with the discussion in this article, there are many factors affecting fusion effect: fusion welding joint shape, fusion temperature, fusion time and layer stackup. Rectangular fusion welding joint brings forward better fusion effect than circular fusion welding joint. At equivalent layer stackup and fusion time, the higher fusion temeprature is, fusion expansion area will be. Too low fusion temperature will lead fusion expansion area to be uneven with crack risks. The longer fusion time is, the larger fusion expansion area will be. When fusion time exceeds 15 seconds, fusion expansion area will be enlarged with bad fusion effect generated. The thinner prepreg structure is, the more even fusion expansion will be. Thus, 2116 or below prepreg is the most suitable for fusion.
