پروفایل دمای دستگاه مونتاژ SMD

1394-1-22 ( 9 سال پیش ) دسته خبری : آموزشی

 در اینجا به شرح مشکلات رایج در مونتاژ قطعات SMD به وسیله کوره های مخصوص (REFLOW OVEN) می پردازیم.

پروفایل استاندارد مورد استفاده برای مونتاژ SMD به صورت کلی به شکل  زیر می باشد.

 

 وظیفه هر مرحله از این نمودار به شرح ذیل می باشد

PREHEAT :   گرم کردن اولیه بصورت تدریجی - در این مرحله تغییر خاصی نداریم و خمیرقلع آماده مراحل بعدی می شود

FLUX ACTIVATON : با رسیدن به یک دمای آستانه ، سرعت گرم کردن کم می شود و فلاکس شروع به فعال شدن و تغییرات شیمیایی می کند.

DRAYING: در این مرحله بخش عمده فلاکس بخار شده و گلوله های ریز قلع شروع به ذوب شدن می کنند

 REFLOW : قلع بصورت کامل ذوب شده و بر روی pad و پایه های قطعات جاری می شود.

COOLING :  بصورت تدریجی ، دما پایین کشیده می شود تا کمترین تاثیر را بر روی قطعات نیمه هادی داشته باشد.

                     

در ادامه به شرح مشکلات رایج و روش جل  آنها می پردازیم  البته برای جلوگیری از هرگونه دخل و تصرف در آن متن انگلیسی ان را برای شما دوستان قرار داده ایم.

                              

                               

The Science behind the Lee Profile

As mentioned earlier, the ramp to dwell ramp to peek (RDRP) profile was developed for infra-red (IR) reflow ovens. This technology tends to heat unevenly and in some respects more slowly than convection ovens. Consequently the dwell, usually at 140 to 160°C, was developed to assure as even as possible heating with IR technology in the solvent evaporation and flux activation stages of the reflow process. The ramp to peak was then employed to minimize the time above liquidus (TAL) and the possibility of singeing components. When IR technology was deployed, 0402 passives, ultra-fine pitched PQFPs, BGAs, CSPs, and other “high tech” components were far in the future. Unfortunately, each of these now dominant technologies has a problem with the RDRP profile. Many of the failure mechanisms, common today with these components, can be traced to the use of the RDRP profile.

Tombstoning

Tombstoning is a phenomenon in which uneven melting of the solder paste causes the surface tension of the melted solder to lift the passive as shown in Figure 3.

 

Figure 3 – The uneven melting caused by the RDPR Profile can produce Tombstones. The Lee Profile has

a brief dwell as the oven passes through the melting point of the solder to minimize this failure mode.

 

This failure mode is almost unavoidable with RDPR. As the RDPR profile goes from the soak temperature of 140 to 160°C, it shoots straight up to the peak reflow temperature. This fast temperature rise from below to above liquidus will often cause the solder paste at one end of a passive to melt before the paste at the other end. The surface tension of the melted solder will cause the passive to tombstone. The Lee Profile minimizes tombstoning by establishing a brief dwell as the profile goes through liquidus. This dwell allows for more even temperatures as solder paste at the component leads goes through liquidus. This profile melts the solder paste at both ends of the passive simultaneously, hence minimizing tombstoning.

Wicking

Wicking occurs when the leads of the components become significantly hotter than the PWB pads during reflow. Since solder flows to where the temperature is highest, opens can result as seen in Figure 4.

 

Figure 4 – If the component leads become much hotter than the pads, Wicking can result causing opens

 

The RDRP profile typically ramps from 1-2°C/s from its dwell. This high ramp rate and the lack of a dwell at liquidus can result in the leads being much hotter than the pads. Wicking will then often follow. The Lee Profile’s more gentile heating rate of 0.5 to 1.0°C and a brief dwell at liquidus help to minimize such wicking.

Solder Balling

Solder balling is an all too common phenomenon today. The RDRP profile can be the culprit. Its rapid ramp rate can cause the solvents to escape so rapidly that spattering of the paste occurs. In addition the long time at a relatively high dwell temperature can result in oxidation. The combination of these two mechanisms can create solder balling. The spattering disperses the solder paste and the oxidation prevents coalescence of the melted solder into the solder joint. The Lee Profile’s gentler ramp rate minimizes spattering and the lack of a long dwell reduces oxidation. Therefore, the Lee Profile has a tendency to minimize solder balling (see Figure 5.) However, stencil design can also be a factor as Dr. Lee points out in his book¹.

 

Figure 5 – The gentle ramp rate and lack of a long dwell, tends to minimize Soldering Balling in the Lee Profile

 

 

Hot Slumping – Bridging

Hot slumping occurs when the solder paste is at too high a temperature for too long a time. Hot slumping can lead to solder bridging. The long dwell of the RDRP profile can result in hot slump. The gentle ramp of the Lee Profile minimizes this failure mode as the solder paste is exposed to high temperatures for a shorter time (see Figure 6.)

 

Figure 6 – The long dwell at high temperature can cause Hot Slumping or even Bridging in reflow.

The gentle ramp of the Lee Profile minimizes this effect.

 

Poor Wetting

The RDRP profile can expose the solder paste and leads and pads to excessive temperature and time. This can cause oxidation which results in poor wetting. The Lee Profile minimizes the time at high temperatures, reducing the chance of excessive oxidation, making good wetting more likely (see Figure 7.)

 

Figure 7 – Poor Wetting can be the result of exposure to high temperature for excessive time

 

Voiding

The combination of oxidation and the flux remnant being too viscous can result in voids. A viscous flux remnant is not able to move through the molten solder an escape through the surface. The longer times at high temperatures that the RDRP profile provides can cause oxidation, as previously discussed. However, this profile can also drive off too much solvent and leave a viscous flux remnant. The shorter time at higher temperatures, provided by the Lee Profile, minimizes the oxidation and leaves the flux remnant fluid enough to flow to the surface of the molten solder, minimizing void formation (see Figure 8.)

 

Figure 8 – The shorter time at higher temperatures in the Lee Profile, Minimizes Oxidation

and leaves a fluid flux remnant, thus Minimizing Voiding

 

Excessive Peek Temperature & Time Defects

Peek temperatures that are too high can result in charring of components. The combination of high peek temperature or excessive time above liquidus (TAL) can also create intermetallics that are too thick, resulting in reliability concerns. The Lee Profile recommends an absolute peak temperature of 228°C, but encourages the user to strive for 215°C as a target peak temperature. The Lee Profile minimum TAL can be as short as 30 seconds. Some RPRD profiles suggest peak temperatures of 235°C and minimum TALs of 45 seconds. These types of profiles can cause charring of components or form intermetallics that are too thick (see Figure 9.)

 

 

Figure 9 – The TAL and Peak Temperatures should be such as to minimize damage

to components and the creation of intermetallics that are too thick

 

Tools to Verify and Control the Lee Profile

To achieve the higher yields described above, it is not sufficient to simply select any oven recipe that provides an in-spec profile. A smaller subset of oven recipes, those that yield a profile that conforms to Lee profile specifications, needs to be identified. For example:

·         From room temperature, ramp at 0.5-1.0°C/s to 175 +/- 3

·         Ramp at <0.25°C through 183° but for less than 45 seconds

·         Ramp to peak (target 215°, range 208°-228°) with TAL 30-90 seconds

·         Ramp down 2-4°C/s.

 

  

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