Process Improvement, the Final-Cure

Reduce cycle-time and curing cost in final-cure by up to 80%, while improving quality.

BY: Dave Corey

“Bringing about or constituting a great or radical change [a revolutionary design].” This definition of the word revolutionary can be applied to the curing process for inks used in the printed circuit board (PCB) industry. The curing process has undergone a great and radical change over the last few years. My experience during this time has been focused on the utilization of infrared technology vs. conventional convection technology. During this process, I have had the privilege of working with the industry’s major ink manufacturers, as well as equipment and PCB manufacturers around the world. The support and involvement that I’ve received from everyone has made this a very informative and fulfilling time. Our goal at ACS Technologies is to provide the PCB manufacturer cycle-time and cost reduction with minimal investment, resulting in an expeditious ROI. From speaking with various PCB manufacturers worldwide, the areas receiving the highest priority are those that reduce waste, reduce cycle-time and cost, and/or increase quality. As competition becomes even greater, attention must be placed on these four items in an attempt to restore balance back towards profitability.

When considering the acquisition of a piece of equipment, the following reservations must be addressed: Will it reduce cycle-time and cost? Will it improve our current process? Will it reduce waste? These are all vital questions when looking into a new technology and understanding what it may bring to the table.

What would you say if you could reduce your Liquid Photo Imageable Solder Mask (LPISM) final-cure process cycle-time by 60% for those who use batch ovens and by 80% for those who use tunnel ovens? Would this be of interest to you? How about if you could use this same piece of equipment for your legend final-cure process, gaining an additional 60 to 80%, for up to a total of 120 – 160% in cycle-time reduction? What if this same piece of equipment could reduce your electricity usage by providing a very short heat-up time giving you the availability of running the curing system only when you need it or if it provided a Stand-By Mode Feature? What if shorter cycle-time reduced your final-curing cost-per-panel on average by 85%? Finally, what if this same piece of equipment would increase the quality of your process? I know what you must be thinking, but I’m stating facts! I have spoken with different board manufacturers that have invested in the ACS TruCure Infrared Curing System who can confirm these statements.   

Quality

As previously mentioned, there have been improvements in the quality of the curing process. For the final-cure process, these improvements include reduction of solder mask breakdown in the nickel gold process, stress relief of warped PCBs, eliminating the need of a UV bump to enhance the structural integrity of the solder mask, and an absolute uniform and overall superior cure. In regard to legend ink, I have spoken to a board manufacturer, who applies their legend ink via an ink jet printer. This company has improved their legend ink adhesion by processing their PCBs through their TruCure system. In fact, they are curing their solder mask and legend ink simultaneously! In addition, tests have been performed on the curing of conductive ink and silver paste products, greatly reducing the cycle-time there as well.

ACS has worked with all major ink manufacturers who have confirmed in their labs what board manufacturers are experiencing during production. They have endorsed and embraced the curing results of the TruCure system as a viable means to cure their products, stating that the TruCure system is compatible with their products. One ink manufacturer went as far as to state, “It not only meets, but exceeds convection curing process in some areas.”

In addition, independent laboratory tests were performed to determine the volatiles remaining in solder mask after the final-cure process. We sent samples of cured solder mask to an independent laboratory, NuSil Technology. Some of the samples were cured in the TruCure system and some were cured in a tunnel oven. The test method used, Weight Loss, was in accordance with ASTM-595, NASA SP-R0022A, and ESA PSS-01-0702. Upon completion of the test, the results indicated that the PCB cured in the tunnel oven resulted in a .20 weight loss, and the PCB cured in the TruCure system, resulted in a .13 weight loss. These numbers confirmed that there were more volatiles left in the solder mask after the final cure process when utilizing a tunnel oven, thus confirming that the TruCure system produced a more thorough final cure.

Moving ahead, I would like to take this opportunity to discuss convection technology. Convection systems blow hot air across the PCBs to cure the LPISM. The PCBs will begin the heat absorption process from the outside edges in, working its way to the center of the each PCB. The outside edges of the PCBs will reach temperature sooner than the middle. As you can imagine, the outside edges of each board will be subjected to excessive heating, resulting in an inconsistent, yet cured, end product. In addition, excessive cycle-time associated with convection curing creates oxidation on the copper surface, attributing to solder mask breakdown in the nickel-gold process.

Batch ovens have an even greater inconsistency. PCBs are racked up and placed in the oven. The PCBs that are on the ends of the rack will reach temperature sooner than those in the middle of the rack. You must also be careful not to let the PCBs lean into each other while racked in the oven. Inadequate space between boards will degrade the final cure; in addition, it will cause them to become warped as well. 

Most hybrid infrared curing systems utilize inexpensive infrared panel heaters in conjunction with convection heat as their curing source. These infrared panel heaters employ resistor wires that run through refractor material and the resistor wire is covered with a quartz cloth material. These infrared panel heaters are cost effective but offer a very inaccurate curing web. They provide very slow response times and lack the overall performance and quality to that of infrared quartz lamps. Though the costs of these infrared panel heaters are very attractive from a manufacturing point of view, they just lack the quality and performance ACS was looking for.

To offer the PCB industry with the most accurate curing web, ACS developed a unique design for their infrared panel housing. In addition, ACS worked with a leading infrared quartz lamp manufacturer to develop quartz lamps that provide uniform heat distribution across the entire width of the conveyor. No other tunnel oven or infrared curing system offers this accuracy. As you most likely are aware, the LPISM final-cure process is not as important as the LPISM tack-cure process which needs a much more accurate curing window. ACS is proud of the extra steps it took in research and development to achieve this accurate curing control for the LPISM tack-cure process.

As you can see in Table A, at the 12” outside edges, there is a significant drop in energy provided by the hybrid systems. The TruCure IR panel held a tolerance of 400º ± 10º F.

Table A

Table A, represents 24” wide infrared panels. 0 represents the center of the infrared panels. Infrared sensors were placed at each distance as shown in Table A. Measurements were taken across the PCBs simultaneously as they exited the curing zone.  

Cycle-Time

LPISM Final-Cure Process

Now for a closer look at the final-cure process cycle-time. Let’s look at what it will take to final-cure a 50-PCB job. The data used to determine the following calculations was obtained from various tunnel oven, batch oven, and TruCure manufacturer’s specifications. This data was also confirmed by various board manufacturers in regard to the specific curing method that they utilize. As for the tunnel ovens and the TruCure systems, by cycle-time, I am referring to the time from when the first PCB is placed on the entrance conveyor, to when the last PCB is taken off the exit conveyor. The PCB size used for this calculation was 18”x 24”x .062” thick. (See Table B)

Tunnel

With the exception of tunnel oven D, which has an extremely long cycle-time, the average tunnel oven, with an operating temperature of 300º Fahrenheit, total cycle-time is approximately 3 hours. It takes 2 ½ hours for the first PCB to exit the system, and an additional 25 minutes for the fiftieth PCB to exit the system. In addition, the first job of the day takes a little more time as you wait for the machine to heat up. Again, time stated represents total cycle-time. In order to calculate the total time correctly, you must include entry to exit conveyor of the system, not how many slots are in the curing chamber. Moreover, if the PCBs are .125” thick or greater, you might have to increase the distance between them, skipping every other slot in the conveyor, which will increase the cycle-time even more. Those in the industry who are currently using tunnel ovens often refer to them as the “black hole”. 

Batch

Assuming there are no PCBs currently in the oven, cycle-time for one batch is 1 hour, plus an additional 30-minute cool-down period. Operating temperature is 300º Fahrenheit.

TruCure System

With the TruCure system the operating temperature ranges from 400º ± 30º Fahrenheit, the total cycle-time is 37 minutes, including entry to exit conveyor. The total amount of time it will take for first PCB to exit the system is 4 minutes. Heat-up time for the TruCure system is less than 5 minutes.

Table B

Legend Ink Final-Cure Process

Just as with the LPISM calculation let’s look at what it will take to final-cure a 50-PCB job as well. Again the data used to determine the following calculations was obtained from various tunnel oven, batch oven, and TruCure system manufacturer’s specifications. This data was also confirmed by various board manufacturers in regard to the specific curing method that they utilize. As for tunnel ovens and the TruCure systems, by cycle-time, I am referring to the time from when the first PCB is placed on the entrance conveyor, to when the last PCB is taken off the exit conveyor. The PCB size used for this calculation was 18”x 24”x .062” thick. (See Chart C)

Tunnel

With the exception of tunnel oven D, which has an extremely long cycle-time, the average tunnel oven, with an operating temperature of 300º Fahrenheit, total cycle-time is approximately 1½ hours. It takes an hour and 15 minutes just for the first PCB to exit the system, and an additional 15 minutes for the fiftieth PCB to exit the system. In addition, the first job of the day takes a little more time as you wait for the machine to heat up. Again, time stated represents total cycle-time. In order to calculate the total time correctly, you must include entry to exit conveyor of the system, not how many slots are in the curing chamber. Moreover, if the PCBs are .125” thick or greater, you might have to increase the distance between them, skipping every other slot in the conveyor, which will increase the cycle-time even more.

Batch

Assuming there are no PCBs currently in the oven, cycle-time for one batch is 30 - 45 minutes, plus an additional 15 - 30-minute cool-down period. Operating temperature is 300º Fahrenheit.

TruCure System

With the TruCure system, the operating temperature ranges from 400º ± 30º Fahrenheit, the total cycle-time is 19 minutes, including entry to exit conveyor. The total amount of time it will take for first PCB to exit the system is 2 minutes. Heat-up time for pure TruCure systems is less than 5 minutes. The TruCure systems work like UV curing systems, with thermal legend inks curing very rapidly. (See Table C)

Table C

Energy Consumption 

Convection systems are designed to maintain a given set point. These systems are required to stay on all the time in order to maintain their set point. Tunnel ovens, in particular, are large systems, which consume a high amount of energy. I have seen some tunnel oven systems that require hours to heat up, some much less. Either way, these systems are expensive to operate due to their need for continuous run-time in order to maintain their set point. As stated before, there are hybrid infrared curing systems that combine infrared and convection technology. Although they utilize infrared technology, the hybrids still must maintain a given set point and stay on at all times as well. They also have long heat-up times like that of tunnel ovens. In addition, adjustments for each zone are not instant due to the type of infrared panel heaters they use. Even when changing from a high set point to a lower set point, hybrid systems have a long cool-down period as well.

The TruCure systems, due to their rapid heat-up time, can be turned off when there are no PCBs to cure. In addition, the system provides an energy efficient Stand-By Mode Feature (SBMF). The SBMF allows the system to automatically change from normal operating settings to a low energy consumption setting during down-time providing tremendous energy savings. Again, heat-up time is less than 5 minutes!

Cost To Final-Cure 50 PCB’s

With consumable product costs being dissected into pennies per square inch, why not dissect your final-curing cost. Table D is a breakdown of the 4 tunnel ovens along with the TruCure Model TC243. As you can see from Table D, along with excess cycle-times, the TruCure offers an 85% decrease in curing cost per panel. 

* 206 x .8 = 164.8 Total KWM x $.0013 KWM =$.21 x 60 seconds = $12.85 ÷ 50 PCBs = $.26 cost per/PCB

Table D

Material Handling

Tunnel ovens are plagued with material-handling issues. With tunnel ovens, you must purchase special features known as “top hanging”, or use special racks to transport thin material. Top hanging systems require a minimum border free of solder mask in order to adequately support the PCBs. In regard to thick material, most tunnel oven systems require a maximum thickness of .185”. If PCBs exceed .185”, you must purchase a conveyor that accommodates thicker material, or use special racks to transport this material. As for Batch Ovens, a manual process – loading PCBs in racks, is about as inefficient as is gets. 

Reduced Floor Space

Tunnel ovens and hybrid infrared systems in particular, require a huge amount of floor space. Some tunnel ovens and hybrid infrared systems are 35 feet long! In addition, tunnel ovens have heights from 8 to 10 feet. The TruCure systems are much shorter and are available in lengths from 5 to 13 feet. With these small footprints, the TruCure systems provide very high throughputs when needed, they don’t require special shipping, nor do they require walls brought down just to get them into your facility. As for full automation, the size of the TruCure systems makes them ideal to run inline with developers. In addition, the TruTack tack-cure systems are ideal to run inline with screen printers, spray and curtain coaters.

ACS IR “Smart Panels” (Patent Pending)

The only downside with infrared curing systems is the learning curve one goes through due to the variety and product mix a PCB manufacturer can be faced with. With the development of the ACS IR “Smart Panels” this downside has been eliminated.

The temperature of the PCB determines the proper curing. The ACS IR “Smart Panels” are designed to measure the temperature of the PCBs as they are being cured, automatically make the necessary changes, while maintaining the required curing temperature regardless of what type of PCB is being processed. There are certain limitations to a specific set point. PCBs that are .250” will most likely need a different set point or conveyor speed.

The TruCure systems utilize the term “set point” in a different manner. When referring to the set point, we state what PCB surface temperature each infrared sensor is to read and maintain during the curing process. It has nothing to do with maintaining the heat within the system like that of tunnel and batch ovens. Again, the TruCure systems are not ovens and do not require the need to maintain convection heat within the system.     

In the model TC243 there are a total of 6 infrared sensors located in zones 2 and 3. There are infrared sensors located in each ACS IR “Smart Panel” and 1 infrared sensor located at the exit of zones 2 and 3 which reads the actual exit temperature of the PCBs. Again, these infrared sensors are reading and displaying on the touch screen the actual temperature of the PCB as its being cured. With the other curing systems that are available on the market today, you have no way of knowing what the real-time true temperature of the PCB has reached during the cure process. 

Maintenance

The TruCure systems are constructed out of number 304 stainless steel. The design and engineering that went into our machines placed emphasis on quality products, ease of operation, and accessibility to all parts in an attempt to provide minimal down-time. It’s a simple process to remove the ACS IR “Smart Panels” from the machine. Both the top and bottom panels are easily accessed without having to disassemble the machine; the top and bottom “Smart Panels” are mounted on a track assembly designed like a drawer that slides out from the side of the machine for easy access to the top and bottom lamps. If the “Smart Panels” themselves need to be removed from the machine, all electrical wiring is mounted with quick pin disconnects and each panel is secured with just four bolts. Oven manufacturers don’t design extra features like these that are intended to reduce maintenance down-time into their products. An excessive amount of time is needed to remove and/or replace a heating element or panel heaters in conventional ovens. Some ovens even require more than one person just to remove service covers! Simplicity and accessibility of a curing system should be taken into consideration before purchasing a curing system. 

 

Conclusion

New equipment and products come along frequently, but when revolutionary equipment and products are introduced, what are you going to do? Staying ahead of the competition is necessary in tod