【 Weak current College 】 how to improve the quality of welding PCB 】

In recent years with the printed circuit board (PCB) packaging density and high density, increasing paste screen printing process-increasing job quality in the noise.

The so-called screen printing quality is defined as the screen printing mesh blockage caused paste is not printing or printing penetration-the network version and the printed circuit board, and the adjacent ground (Land) lead wire (Bridge), and other lethal defects, unable to reach an even, uneven distribution-free, long continuous and stable quality of solder paste screen printing, so the job requires the use of special paste inspection equipment, the amount of the paste printing for a number of quantitative management instead of traditional Visual inspection, Visual inspection and other qualitative management.

In addition to support high-density packaging of electronic components fixation (Mounter) also for high-precision degree revolution, electronic components fixation of dense accuracy achieved ± 50 μ m volume production requirements.

Although electronic components fixation has reached a high level of precision, but actually encapsulated line fully maintain the ability of the conditions are very difficult, so this article will explore using special paste inspection equipment, the amount of the paste printing for a number of management practices, as well as electronic components fixation device easy to confirm.

Reflow oven analysis of solder paste printing

Figure 1 is a printed circuit board (PCB) of reflow furnace (Reflow) manufacturing quality to analysis graph, as shown in the reflow furnace process occur many and welding, package related adverse; figure 2 and Figure 3 are welded, the specific contents of the packaging side at a glance.

From Figure 2 that are adverse to welding to welding and welding, etc. not paste printing side adverse the most, about welding overall adverse to about 80%.

The paste printing side, as shown in Figure 3 dependsprimarily on three important factors for ppm level to make the paste printing stabilized very difficult, at this point to switch to quantitative observation of printing results, you can obtain a high level of stability of welding quality.

Figure 4 is encapsulated by the objectionable content and events, the figure shows the components turn upwards in the highest, among these, the component sets the location of the sources of the powder cause.

X-Y or θ deviation great occasions, laying job components, printing paste not welded components of electrode contacts, single-side electrode by welding zero-crossing time (Zero-crosstime).

Weight lighter chips class of small electronic component will float over, two electrodes there even if contact paste deviation might, in future, based on the component package high-density and lightweight components, necessary to improve the precision components laying position.
Paste printing volume management

Paste printing inspection equipment is generally divided into secondary element check and third element check two, the second element check device only from the surface observation paste printing shape, it can be made according to the results of surface observation area of the data, but cannot get the three-dimensional nature of volume data and paste height data.

Comparison of three element check device can simultaneously access volume, surface area, height, and other data, table 1 is the second element checks and third element check device comparison at a glance.


In the past based on check processing speed, convenience, price, and other reasons, the second element check equipment has been the market mainstream, in recent years, with high density packaging components compact, flat copper foil area paste printing, stereo-shape differences result in fine paste volume, you might have to package quality, fatal problems using three-element checks the device appears to be increasing.

Figure 5 is the problem of pattern description examples, from the side of diagram two paste shapes, you can find the amount of paste printing, however when from the observation above cannot differentiate the difference, in other words the traditional second element inspection equipment for both paste printing quantity may be adjudicated to the exact same shape, so the researchers are using quadratic element and the third element check device for measuring the amount of solder paste printing and confirmed that the second element check device cannot receive data more than expected, finally decided to switch to the third element check device.


Figure 6 is the third element of the solder paste inspection instrument VP100, check the actual appearance of the project are:

Building volume
Building area
? Height
? Deviation
Lead wire?
Exception highlighting?
Volume too much?
Insufficient volume?
And so on. Table 2 is using three-element VP100 detector, laser displacement sensor, microscopes, measuring equipment of the paste measurement results, according to measurement results show the tester's paste and measuring of appearance and shape are almost identical, the third element Tester and laser displacement sensor, you can get the maximum height of the data, but they are quite close.

Then 12 continuous paste printing substrates, using three-element VP100 tester for volume and weight measurement, the result shown in Figure 7. Here the so-called volume (%) is based on network printing cover design information, will find the ideal amount of paste printing as 100% of relative ratio.

The figure shows the volume increase and decrease and solder paste exactly the changes in weight, although it is a complex combination of paste, printing substrate for evaluation, but the results have shown good correlation, confirmed that the third element VP100 Tester in line with the expected measurement characteristics.



Identified by testing capacity VP100 third element tester, followed by production line paste printing capacity compactor test.

Figure 8 is a mass production of printed circuit board (PCB) Paste printing volume (volume) and unevenly (3 σ) of test results, to examine the volume and unevenly (σ), you can confirm changes very little amount of paste printing has repeatedly been printing, volume (%) and unevenly (σ) every 5 times printing times slightly lower, but it is in line with 「introduction every 5 printings, the required cleaning printing mouldVersion 」, figure in the blue curve is printing templates after cleaning the paste printing of test results.
Usually paste continuous printing, metal material printing templates inside the opening area and will gradually accumulate paste, paste printing quantity becomes unstable, to solve this problem, import the foreign manufacturers print the template on the basis of the above cleaning system, the test results confirmed 「introduction every 5 films cleaning printing templates 1 」 job requirements, are you sure to paste printing quality with positive results.


Paste printing process printing mostly use terminals or flat panel support below support below, the substrate is low by printing rubber scraper (Squeegee) pressure, printing templates and printing substrate appear fine skew, the resulting paste printing quality.

Figure 9 is a substrate of settings below support and printing quality, the previous data management baseboard below support of suitability is difficult, switch to the third element, the tester can paste printing content review board below support of suitability.


Figure 10 is the substrate beneath the set and a supported volume production printing power substrate surface paste volume of distribution relationships, yellow, red, warm colors, line represents the paste volume distribution in appropriate locations, blue, green and blue lines lines represents the paste volume distributionin larger location, by the figure below shows that far from the substrate, paste of some increase in volume.

As noted above the use of three-element tester, you can paste printing quantity measurement management baseboard below support of suitability, supported through the substrate below, in addition to localization suitability of management decided printing pressure outside, on the printing speed, contact angle, and other printing conditions set is also very effective, Figure 11 is a paste printing quality to figure.

Check the device can access a wide range of data, this type of equipment through excellent image processing capabilities, providing high precision 3-element appearance and shape.

In order to make the information on the actual effectiveness, to play quality management necessary to fully grasp the inspection device provide data with actual products quality interaction. Paste printing quantity set specification management process, generally can be divided into:

The absolute content management?
The relative weight management?

2. Figure 12 is the absolute amount of management and the relative amount of management's fundamental conceptual graphs, so-called 「introduction absolute volume management 」 is the amount of the paste printing as well as other light interface reliability modeling, and then through the experimental export ensure interface reliability requirements of the minimum amount, last paste printing will this paste printing volume setting to a lower value specification management.

「Cold relative quantity management 」 under substrate paste printing quantity change information to statistical methods for the substrate to normal obtained paste printing, then the amount of clothing this paste printing set to allow the management level.

In other words use 「introduction absolute volume management 」 specification value does not depend on the substrate by a certain amount of data, use 「introduction relative quantity management 」 specification range, take the intrinsic value of data, since the specifications and specification range there is a relationship between 「introduction and 」, so the actual operation as long as the breach of one of the specifications, the result would have been found as well.



The absolute amount of management is to investigate paste printing and joint reliability as well as reliability problem does not occur, and can realize the solder joints of paste.

In addition to the substrate material, grounding (Land) of the material, shape, components deviating fixed at the time of impact, environmental impact, and to have directly related to one of the most direct review content is paste printing quantity and splice strength of some of the information in table 3 is selected, the paste feature overview of paste, respectively:

?-Lead paste
High melting point of lead-free solder paste
Low melting point buildings lead-free solder paste

Three. Then prepared 4 blade thickness difference of printed film (PrintingMask), depending on the amount of the adjustment, paste printing and carry out scheduled substrate paste printing, and then use printing checks device measuring the solder joints (Pad) amount of paste printing. Component standard laying the reflow furnace package, last in accordance with Figure 13 conditions splice strength measurement



Figure 14 is paste printing quantity and splice strength measurement results paradigm, the figure shows that regardless of what kind of solder paste and component combinations, all the way up to the amount of 50% paste printing, splice strength will rise sharply, then with the paste printing volumes grow, splice strength rendering to ease up.

The main reason is a small amount of solder paste printing, most of the paste will form a semi-circular point interface (Fillet), contrast paste printing quantity many occasions, paste extended to around the Terminalabove ground invasion of paste volume increased, resulting in lower splice strength.

According to the above results spreadsheet reliability problem does not occur, and can realize the solder joints in the amount of the minimum qualifying paste.

Here first when calculating the ideal amount of paste printing, i.e. Paste volume 100% of the splice strength, the calculated value as in Figure 15 of ① shown below.

Then paste volume 100% of the junction, judge allowed% reduction of junction strength implements the reverse calculation splice strength necessary to paste printing volume, the result shown in Figure 15 of ② shows.

The splice strength reduction and paste printing quantity into table 4 events, according to table 4 can induce such as paste types as Sn37Pb, paste volume 100% splice strength, keep 90% of the joint strength of paste of 65.97 ~ 71.67%, the amount of solder paste printing range with components and quantity side distribution.

In other words really paste volume 100% of the joint strength, allowed a few percent lower, with splice strength component specifications, baseboard uses, finished product specifications.

In this context develop specifications management ranges, in addition to measuring the distribution of various specifications, you need to set the environment test result of splice strength reductionIngredients, so the researchers develop under the above sequence contains specification management range of print management.

Component management of laying accuracy

Electronic components fixation (Mounter) packaging accuracy confirmation method, most of Fixer measurement package components of coordinates, and the coordinates of a fixed reader data.

Measurement of accuracy, to avoid the components, the influence of the substrate tolerances, generally require qualified description component or substrate.

Measurement accuracy inherent components laying of finished products, it requiresthe use of the components of the measured object or substrate, this appears:

Confirmed fixed filter function?
? Components of finished laying accuracy confirmation inherent

2 application.

However, regardless of application, the biggest problem is the volume meter accuracy and the use of component and description, to determine whether an encapsulated precision ± 50 μ m, volume meter accuracy requirements necessary to correctly measure μ m grade of accuracy.

In addition to improving the reliability of the measurement results are generally possible measurement more samples, in other words confirm Fixer laying accuracy of measurement instrument, called 「introduction precision "and" speed 」.

The easiest way is to have fixed itself, but the same self diagnostic instruments for measuring and precision dual action, precision of objective reliability has been questioned.

Requires precision time most suitable for high-rate microscope, the disadvantage is that measuring the job very time consuming, on the other hand if you use Visual inspection instrument and so video visual recognition tester, measurement time can be substantially reduced, disadvantage is the measurement accuracy is deteriorating.

Fixer precision validation can use already commercialized by private inspection equipment for high precision and short time, various Fixer precision, variation of the measurement, the disadvantage is that private screening equipment components and substrates is restricted.

Based on the above 「introduction of finished components laying precision inherent 」 actual demand, researchers for accuracy, measurement time, operation of the complex have been commoditized inspection equipment, finally decided to use MITSUTOYO development universal (Universal) and measuring microscope, then combine imaging software Figure 16 of measuring microscope system.
Figure 17 is the holder of the Head of components with precision measuring results assurance, packaging accuracy is ± 0.1mm, according to the survey results showed that the Head of piggyback precision in full compliance with the requirements.

Figure 18 is the holder of the Head of the deviation of measurement results, according to the survey results show that the x axis direction HeadNo.2, and HeadNo.2, y axis direction HeadNo.4, slightly off center coordinates, if observation of σ has found whatever axial Head are in line with expectations.

According to the above results judgment holder has no abnormal structures, HeadNo.2, the amount of deviation HeadNo.4 required for 0ffset processing. If you have questions on the structure, typically measured results of σ will hit big.



As mentioned above measure of accuracy, to avoid the components, the influence of the substrate tolerances, requires qualified description component or substrate, so researchers making Fixer precision examination special-purpose printed circuit board, and in order to lay chip class electronic components for fixed gear performance confirm job, Figure 19 is fixed for performance verification for printed circuit board of the actual appearance.

Printed circuit board laying chip class electronic components, and then use a microscope to detect the chip will be fixed data coordinates, and the difference of the measured data as a variation, Cp and calculated according to the sequence, table 5 Ck is the interface of Cp, Cpk and measuring results at a glance.

According to the survey results confirm Cp Cpk values higher than, the judgment holder A body laying precision is abundant, but off center coordinates; Fixer B x axis to the characteristics and the same: A fixer Fixer C CP, Cpk is very close to each other, in accordance with this judgment the center axis does not deviate from the Center coordinates.

In addition Fixer B and D of the x axis of Cp near 2.0, display the precision of the fixture itself is not sufficient. The amount ofvariation computing package, Cp, Cpk can appropriate judgment of precision characteristics, in other words, through measurement for characteristics of all fixed-rate management set of specifications, regular carrying out of the measurement operation, you can maintain the holder's identity.

As mentioned above confirm that the product's inherent components laying accuracy, retainer of precision, but also by the influence of the substrate and component tolerances, especially the various components of the substrate and mass, size error level dissimilar situations are quite common, normal sexual conduct quality management requires a huge work, in theory, it is almost impossible to perform, but contains new electronic components or a new substrate, fixed process can be detected early on.

Confirm that the product's inherent components laying accuracy, and component quality laying position, package are interactivity is important, it is not a fixed data and measured data the amount of difference, but the component terminals and substrate grounding (Land) of the location relationship measurement, Figure 20 is the actual products Terminal and grounded relative coordinate measurement results.

Use the above methods and measuring of all components of the substrate, is actually quite difficult, so the researchers for the new model component as well as several laying deviations will also affect the quality of packaging, such as micro-chips class component, large quad flat package (QPF: QuadFlatPack) components, connectors (Connector) components, or very close to each other on the design of components and measuring.

According to measurements show the new version of the component, in addition to the manufacturing process can be fixed in advance predictive risk, you verify the coordinate measurement techniques suitable for applications in the fixed (Bond) process, the future can even switch in the automatic dispensing machine (Bonddispenser).

Conclusion

> To achieve high-performance electronic machines, and researchers to printed circuit board components packaging density, as well as component miniaturization of welding quality improvement for the ultimate purpose of reflow furnace manufacturing quality to have a significant effect 「introduction paste printing quantity management "and" components laying position precision management 」 review and various measurements.
Paste printing volume management is in the process, management specification range set to 「introduction absolute amount "and" relative amount 」, components laying position precision management first confirm the fixed component, and then set the components laying position precision management practices.

In addition, researchers will address 「introduction paste printing quantity management "and" components laying position precision management 」, production support validation, take this make printed circuit board Assembly welding adverse to zero.