How to Designing a Successful HDI PCB
2018-12-04 14:55Writer: qyadminReading:
HDI, brief for High Density Interconnection, is a kind of printed circuit table technology beginning to develop by the end of 20th century. For traditional PCB planks, mechanical drilling is used, with some drawbacks including high cost with aperture being 0.15mm and difficulty in improvement consequently of the impact of drilling tool. However, for HDI PCBs, laser beam drilling is used and when it was launched, it received substantial popularity. HDI planks are also known as laser planks whose aperture is typically in the number of 3.0-6.0mil (0.076-0.152mm) and collection width 3.0-4.0mil (0.076-0.10mm), which leads to the actual fact that the pad size can be drastically decreased so that more design can be arranged in each device area. HDI technology adapts and pushes the introduction of PCB industry and today HDI circuit planks have been massively applied in all types of devices.
In conditions of panel design, weighed against ordinary PCBs, the fundamental difference is that HDI PCBs obtain interconnections through blind openings and buried openings rather than through openings. Besides, finer range width and smaller spacing are found in HDI PCBs design so the space for design and monitor can be completely used. Therefore, HDI design novices got to know how to set up element space, how to change applications of blind openings, buried openings and through openings, as well as how to disperse space for transmission lines. Nevertheless, the first and uppermost job is to comprehend the related process guidelines in the HDI PCB production process.
Aperture ratio must be looked at in through opening and blind/buried gap design. For traditional mechanical drilling utilized by regular PCBs, through opening aperture should become more than 0.15mm and plank thickness-to-aperture ratio more than 8:1 (In a few special situation, this parameter can be 12:1 or even more). However, for laserlight drilling, the aperture of laser beam hole should maintain the number of 3 to 6mil among which 4mil is preferred and the plating filling gap depth-to-aperture ratio should be 1:1 for the most part.
The thicker the table is, small the aperture is. Along the way of plating, it's problematic for chemical treatment for enter the depth of drilling openings. Although circuit plating devices press the perfect solution is to the guts of drilling openings through oscillation or pressing, focus gradient makes central plating relatively slim, which leads to slight circuit starting on drilling opening layers. What's worse, when voltage raises or planks suffer impact in severe environment, defects are more obvious, that may finally cause circuit break and panel failing. Therefore, PCB designers must completely find out about the technique features of PCB manufacturers in advance, or PCB fabrication troubles will be added, scrap rate increased or even failing to fabrication.
Planks are usually affected by pressure and temperature along the way of stacking up and there continues to be stress in the planks. If stacked planks are asymmetric, that is, stress distribution on both edges of plank is non-uniform, one-side warpage will be created, drastically decreasing table produce. Therefore, designers must grab asymmetric stack design and distribution of blind/buried openings must be looked at.
• Process flow
Process circulation will be talked about with 4-coating HDI with 1 stacking and 6-level HDI with 2 stacking as good examples.
a. 4-coating HDI with 1 stacking. The next figure shows the procedure movement of 4-level HDI with 1 stacking.
The procedure stream of 4-coating HDI is quite similar with an common PCB except the series of drilling openings. First comes mechanical drilling buried openings of 2-3 level, then comes the mechanical through gap of 1-4 coating, then come 1-2 blind opening and 4-3 blind gap.
If designers straight drill 1-3 opening or 4-2 gap without 2-3 transformation just predicated on design necessity or performance, this design provides extreme problems to fabrication, resulting in the increasing of creation cost and scrap rate. Therefore, when picking right up approach to through openings, present technique and fabrication necessity must be looked at.
b. 6-level HDI with 2 stacking. Determine 6 shows the procedure circulation of 6-coating HDI with 2 stacking.
The procedure movement of 6-level HDI with 2 stacking is comparable with this of a typical PCB except the series of drilling openings. First comes mechanical drilling buried openings on 3-4 coating, then comes drilling of buried opening on 2-5 level, then blind openings on 2-3 coating and 5-4 level, then through openings on 1-6 and last come the drilling of 1-2 blind gap and 6-5 blind opening.
Laser beam drilling is utilized in blind openings on HDI planks and the temperature along the way of laserlight drilling cauterizes the gap wall structure, which generates coke button that is assimilated on the opening wall. At the same time, the high-temperature cauterization can make the copper on the next coating oxidized. Therefore, after laser beam drilling, pre-operation should be applied before electroplating. As the aperture of blind openings is relatively small, it's difficult to remove the coke button. For 2-HDI, professional blind gap plating and filling must be needed, which substantially escalates the cost.
What's worse, the procedure stream of 6-level HDI with 2 stacking needs some counterpoints so the accumulated counterpoint mistakes will rise, and product scrap rate will rise. Therefore, 2-HDI isn't recommended aside from those extremely advanced products.
Component design on HDI planks usually has such high density that it is essential to ensure later installability, solderability and maintainability. The spacing between components can be decided based on the next table.
Components on HDISpacing
Regular SOP & PIN of other components> 40mil
BGA & PIN of other components> 80mil
PIN in normal components> 20mil
Parameters outlined in Desk 1 are limited ideals meeting regular soldering needs. In actual creation , with the concern of installability and maintainability, spacing should be bigger as large as possible within the area to allow easy assembly, rework and soldering.
You will find other aspects that require to be studied under consideration in conditions of design:
• RF/analog/analog-digital transformation/digital part must be purely divided in space. Spacing between them should be bigger no matter they may be on a single part or different edges.
• Layout of the same component should be organized on a single side to be able to reduce the region of drilling or coating changing. So along the way of design, key circuits must be identified and based on the significance degree of signals, they must be organized around key components.
• High-power indicators should be organized definately not other signals.
Some elements must be studied into consideration along the way of tracking like the minimal series width, control of safe spacing and uniformity of monitor. If spacing is too brief, clipping film can be triggered in the internal dried out film process. The residue of film may cause brief circuit. If the collection width is too small, the film has so poor absorption that open up circuit will be triggered. The non-uniformity of circuit will lead to the non-uniformity of copper thickness distribution and resin flowing velocity at different factors. Therefore, monitor and copper non-uniformity must be seen in the procedure of design.
You can find other aspects that require to be studied under consideration in conditions of monitor:
• Components at the top and Bottom level layers have relatively good isolation impact, so shared crosstalk between internal layer indicators should be reduced.
• For important indicators in RF and analog areas, brief ground-to-ground reflow route should rest assured around each transmission.
• Blind openings without physical connection interference mustn't be put into the track part of important indicators in order to avoid the shared interference but blind openings belonging to floor net can.
• Due to the limited space for tracking, it's first and primary for the tracking space of important indicators and indicators with impedance requirements.
• Around the premise of power's capability to meet current, monoblock pavement should be avoided to be able to lessen the interference to other indicators.
• If transmission quality comes first, blind openings can be straight drilled on the pad. For BGA area, blind openings and pad can be held tangent to steer clear of the impact to BGA soldering.
• Blind/buried openings are worse than through openings in conditions of continuity. Therefore, for indicators with impedance necessity, business lead lengths between pad and blind openings, blind openings and buried openings should be shortened and top and lower surface of indicators should be complete.
To conclude, along the way of HDI planks design, complicated manufacturability must be completely considered beforehand. Process guidelines of regular PCBs have been familiar to many designers while HDI design novices ought to know about HDI PCB specification of custom circuit panel manufacturers to be able to ensure their tasks.